JP2000111855A - Multiple-passage type acoustooptic device and laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate many problems and risks of manufacture when the temperature and stability are balanced between two stages by providing the acoustooptic switch including a 1st and a 2nd polarized-light conversion area which are both coupled among 1st to 4th optical ports and an optical combination including an optical insulating element coupled between the 2nd and 3rd optical ports. SOLUTION: The acoustooptic switch 8 is a 2×2 filter which responds selecting wavelength without depending upon polarization. Two input ports 202 and 203 and 1st and 2nd polarized light conversion areas of upper and lower converters U and L coupled between the two output ports 204 and 205 are included. The optical combination including the optical insulating element of an optical isolator 7 coupled between the output ports 204 and 205 is provided. For example, a cross mode is selected with a control signal, the 1st input port 202 is connected to the 1st output port 205 and the 2nd input port 203 is connected to the 2nd output port 204.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double-pass acoustic optical device.
Optical devices, i.e. acousto-optical devices through which light passes multiple times, in particular various optical filters,
Wavelength addition / drop device (wavelength)
The present invention relates to a laser configured using an add / drop device, particularly a two-pass optical device.

[0002]

BACKGROUND OF THE INVENTION It is known to use optical fibers for transmitting optical information-carrying signals for long-distance communications.

[0003] Optical telecommunications systems using wavelength division multiplexing (WDM) transmission are known. In these systems, several channels, i.e., many independent transmission signals, are transmitted on the same line by wavelength division multiplexing.
The transmission channels can be digital or analog and each of these channels is identifiable because it is associated with a particular wavelength.

[0004] Applicants have observed that known WDM communication systems are limited with respect to the number of channels, independent wavelengths available for transmission within the wavelength band available for signal transmission and amplification.

In order to combine and separate signals having different wavelengths, that is, to combine signals at a transmission station, for example, a signal is given to a receiver located at an intermediate node of the line, or another signal is sent to the intermediate node. , Or to transmit such a signal to separate the receiver at the receiving station, adjacent channels (with respect to wavelength) must be separated by more than a predetermined minimum.

The minimum is determined by the spectral characteristics of the wavelength-selective components (eg, bandwidth, center-band attenuation, minimum sensitivity), and by the selective components themselves and the optical signal source (thermal and temporal). It depends on the characteristics of the components used in the system, such as the wavelength stability.

In particular, Applicants have found that the spectral selectivity of currently available wavelength-selective components increases the signal selectivity in the multi-channel transmission spectrum, especially when there are signals with closely spaced wavelengths, eg, less than 2 nm. Addition (addi
ng) and the possibility of dropping could be very limited.

[0008] In optical filtering for optical communication systems or other purposes, the filtering performance is increased compared to a single-stage filter having the same characteristics, so that a double-stable filter is used.
stage) optical filters are advantageous. Acousto-optic filters are known that produce an interaction between an optical signal propagating in a waveguide formed on a substrate of birefringent and photoelastic materials and a sound wave propagating on the substrate surface. This sound wave is generated by a suitable transducer and is initially provided by radio frequency.

The interaction between the polarized light signal and the sound wave is a polarization transformation of the signal, in other words, from its mutually orthogonal electrical transverse TE component to its magnetic transverse TM component.
And the opposite polarization change. Following such interaction with the sound wave, the polarization component is not only converted to the corresponding orthogonal component, but also a frequency shift whose absolute value is equal to the frequency of the interacting sound wave (and thus equal to the frequency of the applied radio frequency signal). Also occurs. The sign of this frequency shift is a function of the state of polarization and the direction of propagation of the sound wave relative to the light wave.

In addition to their use as pure filters, acousto-optical devices have also been used as tuning devices for lasers. However, to the applicant's knowledge, conventional acousto-optic devices used as filters or tuners for lasers typically use a single-stage acousto-optic substrate using a single pass of the light of the device, or separate physical Cascaded acousto-optic filters constructed on a static substrate.

For example, European Patent Application No. 97 11
No. 3188.3 discloses a substrate made of a material capable of propagating a surface sound along a part of the surface of the substrate, a transducer for generating the surface acoustic wave, an optical waveguide formed on the substrate, Acoustic absorber surrounding part of the substrate
rber).

EP 0 814 364 A1 describes a two-stage acousto-optical waveguide device. FIG. 12 shows a switch or add / drop node that includes a fourth polarization conversion stage 403 in addition to the third polarization conversion stage 303. This fourth polarization conversion stage 403
Are input polarization splitter 404 and output polarization splitter 4
05. Further, the splitter 405 includes:
The connection branch 210 connects to the polarization splitter 204 and the transverse waveguide 255. Ports 19, 20,
21 and 22 are connected to the line. The polarization splitter 404 has an input port 25 where the add and drop signals are introduced and introduced via ports 23 and 24.
And 26.

US Pat. No. 5,611,004 describes a tunable acousto-optic filter (AOTF) that does not involve polarization. The patent discloses that in FIG.
An embodiment in which two stages of signal filtering are performed by only one transducer 43 on 1 is described. 2
The filtering of the stage initially involves the incident light beam being AOTF
To reflect the light beam from the mirror 67,
The second is implemented by passing the light beam through the same AOTF. The bandpass filtered state of the original light beam is obtained at the circulator output 71 of the optical circulator 69 located at the input of this embodiment.

US Pat. No. 5,452,314 describes a tunable acousto-optic filter having a pair of electrodes on either side of the waveguide. The patent discloses the use of a voltage source whose applied electric field controls the birefringence of the filter, and a tunable laser including a tunable acousto-optic filter and the like. By properly adjusting the potential applied by the voltage source,
The result is suppression of sidelobes, correction of asymmetric sidelobes, and compensation of physical aspects in the waveguide.

US Pat. No. 5,002,349 and EP 805372 describe a single converter tunable acousto-optic filter. U.S. Pat. No. 5,002,349 discloses such a multi-stage converter with two stages of zero frequency shifted converters and filters, lasers using acousto-optic filters as tuning elements, polarization independent converters, and wavelength separation multiplexing. An acousto-optical converter is disclosed that enables to provide a routing switch.

D. A. Smith et al., “Acoustically tuned erbium-doped fiber ring lasers.
um-doped fiber ring race
r) "(Optics, Transactions, Volume 16, Part 6, 199
(March 15, 1) describes a continuously tunable laser that uses an acousto-optic filter to achieve a wide tuning range. I. "Acoustic Tunable Ti: Er: LiNbO Waveguide Laser (Aco
conventionally tunable Ti: Er. L
iNbO waveguide laser) ”(ECOC '94, 4th
Vol. 99-102) describes a waveguide laser that includes a two-stage waveguide filter used as a narrowband tunable optical amplifier.

WO 98/11463, with reference to FIG. 7, is an embodiment in which it is possible to reduce the polarization sensitivity of a null coupler tunable acousto-optical filter by applying two sound waves simultaneously. Is disclosed. A null coupler is made from two optical fibers whose diameters are matched so that the resulting coupler produces very little passive coupling efficiency. The coupler is made by pre-tapering one of two identical single-mode fibers along a short distance before both fibers are melted and stretched together to form a coupler.

In WO 98/11463,
The input light in a fiber that is not pre-tapered is
In the narrow region of the coupler, only the fundamental mode is excited. Light in other fibers excites only the second mode. When a sound wave matches the optical beat length of two modes in a narrow region, a resonant coupling occurs between them. After light is incident on port 1 of the device, both polarization components are coupled to port 2, but produce a different upward shift in frequency.
After propagating through the isolator loop, which is not involved in polarization,
Each polarized light reenters the combiner and is coupled a second time and exits port 4 at the downshifted frequency.

In contrast, devices that use a polarization converter as opposed to a mode converter as in WO 98/11463 use two separate waveguides of birefringent material rather than a fused fiber coupler.
In a fiber coupler configuration, the optical signal passes through only a single waveguide and conversion region. No. WO 98/114
The mode-type coupling device in No. 63 also faces high polarization sensitivity, which requires two separate radio frequencies to overcome.

Applicants have discovered that a two-stage acousto-optic filter with a polarization converter as opposed to a mode converter provides a lower than desired level for the optical signal due to the physical layout of the configuration. In particular, Applicants have observed that it is difficult to match the filter characteristics of a two-stage filter with a conventional two-stage filter, especially when the filter stages are physically separated. Similarly, Applicants have found that matching the temperature of the two stages presents a problem as well. Furthermore, two separate stages require a separate driver frequency for each of their acousto-optic devices that may be mismatched and must therefore be individually calibrated.

[0021]

Applicants have noted that the amount of filtering achievable by an acousto-optic filter is highly dependent on the configuration chosen for the filter and its components. In particular, Applicants have found that single pass and single stage acousto-optic filters exhibit inadequate filtering performance. Similarly, a two-stage filter integrated on one substrate creates many manufacturing problems and risks in properly balancing the temperature and stability between the two stages.

[0022]

Accordingly, Applicants have developed an inventive acousto-optic filter and laser using filters for multiple pass and single stage operation. The filter of the present invention has a single temperature control (no temperature difference between the two stages), a single radio frequency control, halving the electroacoustic transducer and radio frequency power for a two stage device, miniaturization, narrow bandwidth, and notch Enables compatibility with filters with output and use as add / drop devices. Similarly, the filter eliminates frequency shifts and wavelength shifts as needed to implement a device that does not participate in effective polarization, and eliminates frequency shifts even when the radio frequencies in the two converters are different. Offer the possibility.

In one aspect, the multi-pass acousto-optic device comprises a first, second, third, and fourth light with an optical coupling coupled between second and third optical ports including an optical isolation element. An acousto-optical switch including first and second polarization conversion regions coupled between the ports. Preferably,
The first optical splitter is disposed between the first and fourth optical ports and between the first and second polarization conversion regions, and each of the first optical splitters has a cross / quadrature for orthogonal polarization components of the received light. Bar transmission (cross and bart)
transmission). Preferably, the second optical splitter is disposed between the second and third optical ports and the first and second polarization conversion regions,
Here, it is desirable that each of the second optical splitters generates cross / bar transmission for orthogonal polarization components of the received light.

More preferably, the two-pass acousto-optical device according to the present invention includes an upper transducer in an acousto-optical switch acoustically coupled to the first polarization conversion region and coupled to an RF source. The transducer has a first frequency having a characteristic frequency determined by the RF source.
A first sound wave is generated in the polarization conversion region of. Similarly, 2
The single pass acousto-optic device further includes a lower transducer in an acousto-optic switch acoustically coupled to the second polarization conversion region and coupled to the RF source, wherein the lower transducer is in a propagation direction of the first sound wave. A second sound wave is generated in a second polarization conversion region having a characteristic frequency having a propagation direction opposite to that of the second sound wave.

In another aspect, a tunable laser generator using an acousto-optical device comprises a first, a second, a third, and a fourth.
An acousto-optical switch having first and second polarization converters coupled between the first and second optical ports, and a first optical half-ring including an optical amplifier coupled between the first and fourth optical ports. and a second optical half ring coupled between the second and third optical ports and including an optical isolation element. The laser includes a laser output coupler located within the first optical half ring. Desirably, the tunable laser generator has an optical isolation element in the first optical half ring. Preferably,
One or both of the first half ring and the second half ring comprise a polarization maintaining element. Alternatively, one or both of the first half ring and the second half ring include a polarization controller.
Desirably, the RF generator is coupled to the first and second polarization converters for tuning the laser generator.

Preferably, the tunable laser generator further comprises:
A first optical splitter disposed between the first and fourth optical ports and between the first and second polarization converters, wherein the first optical splitter includes a cross / bar for each orthogonal polarization component of the received light; Produce transmission. Preferably,
The laser generator further includes a second optical splitter disposed between the second and third optical ports and between the first and second polarization converters, each of the second optical splitters being orthogonal to the received light. Cross / bar transmission occurs for each polarization component.

Preferably, the tunable laser generator further comprises:
An upper transducer in an acousto-optic switch acoustically coupled to the first polarization converter and coupled to the RF source, the upper transducer having a characteristic frequency determined by the RF source in the first polarization converter. Generates sound waves. In one embodiment, the transducer further comprises a lower transducer in an acousto-optic switch acoustically coupled to the second polarization converter and coupled to the RF source, wherein the lower transducer comprises a first transducer in the second polarization converter. A second sound wave having a characteristic frequency having a propagation direction opposite to the propagation direction of the sound wave is generated. According to different alternative embodiments, the first and second polarization converters are arranged with sufficient approximation that the first sound wave travels in both the first and second polarization converters.

In another aspect of the invention, an optical frequency filtering method using an acousto-optical device includes a first and a second polarization converter coupled between first, second, third and fourth optical ports. At least one acousto-optical switch including a plurality of wavelengths, injecting an input optical signal having a plurality of wavelengths into a first optical port, and optically isolating an intermediate optical signal including a subset of the plurality of wavelengths from a second optical port. Feedback to the third optical port via the element and extracting an output optical signal from the fourth optical port that includes a subset of the plurality of wavelengths. Desirably, the method further comprises the step of extracting the remainder of the plurality of wavelengths from the second optical port via an optical isolation element, wherein the optical isolation element is an optical circulator having at least three ports. Alternatively,
The method further includes the step of inputting the additional wavelength to the device via the optical isolation element. Alternatively, the method may further include controlling the polarization of the intermediate optical signal, or maintaining the polarization of the intermediate optical signal using a polarization maintaining component.

More preferably, the method according to the present invention comprises splitting the input optical signal into a TE component and a TM component in a first optical splitter in the switch, sending the TE component to a first polarization conversion area, Sending the component to the second polarization conversion region, performing quadrature phase conversion of the TE initial component of the input optical signal having the selected frequency to a TM intermediate component in the first polarization conversion region, and inputting the input optical signal having the selected frequency And the quadrature phase conversion of the TM initial component to the TE intermediate component in the second polarization conversion region. Alternatively, the method further comprises combining the intermediate components of TE and TM into an intermediate optical signal, or combining the initial components of TE and TM into an intermediate optical signal.

Preferably, the method further comprises, after the feedback step, splitting the intermediate optical signal into a TE and TM feedback component in a second optical splitter in the switch, and dividing the TE feedback component into a first polarization conversion region. Sending the TM feedback component to the second polarization conversion region, quadrature-phase converting the TE feedback component having the selected frequency to the TM final component in the first polarization conversion region, and transmitting the TM feedback component having the selected frequency. To the quadrature phase conversion into the TE final component in the second polarization conversion region. Alternatively, the method further comprises combining the TE and TM final components into the output optical signal or combining the TE and TM feedback components into the output optical signal.

Applicants note that in the two-pass acousto-optical device according to the invention, the filter characteristics are always matched, there is no need for temperature matching, and only one driver frequency is required for the acousto-optic filter.

It is to be understood that the above general description and the following detailed description are exemplary only, and not restrictive of the invention, as set forth in the following claims. The following description sets forth and suggests further advantages and objects of the present invention, as well as the practice of the present invention.

The accompanying drawings, which are incorporated in and constitute a part of this specification, together with the text, illustrate the advantages and principles of the present invention.

[0034]

BRIEF DESCRIPTION OF THE DRAWINGS The invention will first become apparent from the description of the invention with reference to the various embodiments of the invention shown in the accompanying drawings. In the drawings, the same reference number represents the same or similar element in different drawings wherever possible.

As shown generally in FIG. 1, a two-pass acousto-optical device according to the present invention comprises a first polarization converter and a second input / output port connected between first input / output ports. An acousto-optical switch having a second polarization converter connected therebetween and a photosynthesis unit including an optical insulating element connected between the first output port and the second input port.

The two-pass acousto-optical device disclosed hereinafter is an optical communication system, in particular, a plurality of light sources for generating optical signals of different wavelengths, and a plurality of signals transmitted to a normal optical path including an optical fiber line. Used in a WDM optical transmission system that includes a combiner and a signal optical receiver. 2
Single pass acousto-optic devices, for example, separate (filter) one or more channels from the remaining transmission channels and / or noise, and separate (drop) signals with different wavelengths into different optical paths. drop),
Alternatively, an optical signal at one or more selected wavelengths can be input to an optical fiber (addition), so that it can be used in the optical transmission system described above.

In FIG. 1, the acousto-optic device, generally indicated as 10, is an input light into which light radiation is introduced from an external light source (not shown) connected to the first input port 202 of the acousto-optic switch 8. Fiber 3 is included. The switch 8 is connected to the radio frequency generator 6. The first output port 205 of the acousto-optical switch 8 is connected to the optical isolator 7 whose output is optically connected or coupled to the polarization controller 9. The output of the polarization controller 9 is connected to the second input port 203 of the acousto-optical switch 8. The second output port 204 is connected to the output fiber 2 of the acousto-optic device 10. The term "coupled" means that two physical devices are joined by a common optical path, and possibly but not necessarily, physically attached. Applicants have used the terms “coupling” and “connection” interchangeably in the description of the present invention,
It will be appreciated that the various components shown herein need not be physically attached to each other to provide optical coupling that helps achieve the advantageous results of the present invention.

The optical fiber linking the various components described herein is a single mode optical fiber of the type 9/125, where 9 is the core diameter and 125 is the fiber cladding diameter in microns. Fiber may be used. An example of such a fiber has its core doped with germanium and has a numerical aperture (NA) of about 0.13. Other known transmission fibers are also sufficient to transmit optical signals to the acousto-optic device 10.

The polarization controller 9 includes a plurality of optical fibers supported in sequence so as to provide the desired polarization control and can be oriented with respect to a common alignment axis. Devices of the type mentioned above are available from Northerns UK
Towcester, Caswe of (NN128EQ)
ll of GEC MARCONI MATERIALST
Commercially available from ECHNOLOGY. A polarization controller suitable for use in the present invention is HP-11
896 and HP-8169. However, those skilled in the art will appreciate that other controllers equivalent to those described above for adjusting polarization may alternatively be used.

The optical isolator 7 is preferably of a type having an insulation degree of more than 35 dB and a reflectance of less than -50 dB without depending on the polarization of a transmission signal passing through the acousto-optical switch 8. In particular, sufficient isolators for device 7 are Newport ISO 1
5FTB. However, those skilled in the art will appreciate that other isolators equivalent to those described above may be used. The optical isolator 7 can be used to set the traveling direction of radiation in the acousto-optical device 10.

The radio frequency generator 6 for driving the acousto-optic filter has a wavelength of the output light radiation (for example, 1515 nm <λ <
1565 nm), for example, with a frequency that can vary between 160 MHz and 180 MHz.

Acoustic-light switch 8 in device 10
Is a 2 × 2 filter selected for wavelength and providing a polarization independent response. The switch 8 can be used to select an optical signal between two inputs 202, 203 and two outputs 204, 205 according to its wavelength and the value of the appropriate control signal.

A control signal (not shown) can change the transmission state of switch 8 for a wavelength selected in one of two modes. First, Figure 1
, A cross mode connecting the first input port 202 to the first output port 205 and connecting the second input port 203 to the second output port 204 can be used. Alternatively, the first input port 202 is connected to the second input port 203 and the second output port 2
A bar mode can be used that connects 04 to the first output port 205. The acousto-optic switch 8 also allows the radiation input port to be exchanged with the output port in both the bar mode and cross mode directions as described above without changing the operation of the filter.

The operation of the integrated acousto-optical device is accomplished by generating an optical signal propagating through a waveguide formed on the substrate of birefringent and photoelastic materials and a suitable transducer provided by a radio frequency signal. It is based on the interaction between sound waves propagating on the surface. The interaction between the polarized light signal and the acoustic wave is the polarization transformation of the signal, in other words the change of polarization from its mutually orthogonal TE (electrical transverse wave) component to the TM (magnetic transverse wave) component, and The opposite change occurs. Following such interaction with the acoustic wave, the polarization component undergoes a frequency shift as well as a conversion to the corresponding orthogonal component. Such a shift has an absolute value equal to the frequency of the interacting sound wave (and therefore equal to the frequency of the applied radio frequency signal). Similarly, the frequency shift is
It has a sign that is a function of the direction of propagation of the sound wave with respect to the polarization state and the light wave. Table 1 summarizes these relationships.

[0045]

[Table 1]

The radio frequencies used to feed the two transducers inside the switch 8 are the same, so that the frequency of the sound waves and the frequency shift induced in the two converters are the same. Thus, both transformed polarizations from a single pass of the filter exhibit equal frequency shifts, ie + f _RF or −f _RF , and the sign depends on the input port (in FIG. 1, to be +, port 204 or 203, − Port 202 or 205).

In these acousto-optic filters, it is possible to tune the spectral response curve by controlling the frequency of the sound waves, which modifies the signal selection without changing the component windings. Enable. If the sound waves propagating on the substrate surface are a superposition of different sound waves, these acousto-optical elements can also be used to switch and simultaneously select the emission at different wavelengths. Such switching and selection can occur because the acousto-optic filter produces a combination selection of signals of wavelengths corresponding to the frequencies simultaneously applied to the electrodes of the electroacoustic transducer.

An acousto-optic switch 8 in one particular embodiment suitable for the purpose of the present invention is shown in FIG. The filter includes a substrate 101 made of a birefringent material made of lithium niobate (LiNbO ₃ ) and a photoelastic material. Two optical waveguide branches 102, 103 connected to two ports 202 and 204 are formed in the substrate 101. The two polarization elements 104, 105, the converter stage 108, and the two output optical waveguide branches 106, 107 connected to the ports 203, 205 comprise the substrate 10
1 is formed.

The optional polarizing elements 104 and 105
Desirably, it comprises a polarization divider formed by a directional coupler with zero-spaced waves. These components can split the two corresponding polarizations provided to a common input into two output waveguides and separate the two corresponding polarizations provided to two separate input waveguides into a common output waveguide. Can be combined with a waveguide. In particular, polarizing elements 104, 105
Are respectively central optical waveguides 109, 110 and input / output waveguides 111, 112, 113, 114, respectively.
115, 116, 117, and 118 corresponding pairs. These optional polarization elements are symmetric, in other words,
If input optical waveguides are used as output waveguides or vice versa, their behavior does not change.

The conversion stage 108 inside the switch 8 includes two conversion zones, referred to in the text as upper converter U and lower converter L. Upper converter U
Includes an optical waveguide branch 119 connected to the output waveguide 115 of the polarization divider 104 and the input waveguide 114 of the polarization divider 105. The upper converter U also includes an acoustic waveguide 121. The lower converter L includes an output waveguide 116 of the polarization divider 104 and the polarization divider 10.
5 includes an optical waveguide branch 120 connected to the fifth input waveguide 113. The lower converter L is also the acoustic waveguide 12
2 inclusive.

The sound center cladding 129 is the acoustic waveguide 1
21 and 122 are divided. The two piezoelectric transducers 123, 124 connected by a radio frequency splitter (not shown) to the radio frequency generator 6 in FIG.
It is formed by a pair of interconnected electrodes. Transducers 123 and 124 are capable of generating radio frequency surface acoustic waves and have acoustic waveguides 125 and 12 respectively.
6 inside. Acoustic waveguides 125 and 126 are each configured to form an acoustic coupler.
1, 122.

Electroacoustic transducers 123 and 124
Generated by the optical waveguide branches 119, 1
Propagating in opposite directions along corresponding parallel waveguides 121, 122 so as to interact with electromagnetic waves propagating in 20. Next, the sound waves are transmitted through the acoustic waveguides 127 and 128, respectively.
Is connected to each part. An acoustic absorber 130 at the end of the acoustic waveguides 127, 128 blocks the reflection of sound waves.

European Patent Application No. 971 in the name of the applicant
No. 13188.3 describes an acousto-optical device suitable for use, for example, in the device shown in FIG. An acousto-optic filter suitable for use in the present invention and of the same type as described above is, for example, PIRAOS-150X manufactured by the applicant. Typically, a plurality (typically four) of tunable 2 wavelengths are selectable.
× 2 switches are configured in parallel on the same substrate and can be used independently. In addition, the terminating surface (ie, the surface connected to the port of the acousto-optical switch 8) is polished diagonally (about 6 °) or has an anti-reflective coating to avoid back reflection. Of course, devices that are operatively similar to the acousto-optical switch 8 of the present invention may alternatively be used.

FIG. 3 is a graph of a characteristic curve of the PIRAOS-150X switch. From FIG. 3, it can be seen that the depth of the cut-off curve, called the notch, is at least 18 dB, the filter selection curve has a peak of 20 dB, both 1545 nm (for the specific radio frequency supplied).
At the center frequency of the band. The operation of the device depends on the operating temperature and therefore
A thermal conditioning system using a Peltier cell (not shown) is provided for each waveguide where polarization conversion occurs. PIR
AOS-150X devices are typically kept at a stable temperature of 25C.

Another characteristic parameter of these devices is:
PDL (polarization dependent loss), which represents the difference between the losses that occur in the switch as a result of two orthogonal polarizations TE and TM. Additional characteristic parameters of the PIRAOS-150X acousto-optic filter are described below.

Switching frequency: 174.9 MHz
RF power: 18.6 dBm; Max loss: 6 dB PDL: <0.5 dB Leakage: <-20 dB Tuning slope Δλ / Δf: −8.2 nm / MHz Temperature coefficient Δλ / ΔT: −0. 9 nm / ° C. Switching bandwidth at −3 dB: 2.3 nm Switching bandwidth at −10 dB: 4.2 nm Sidelobe level: <−19 dB The two converters L and U, in implementation, for a given radio frequency value The absolute value of the wavelength at which conversion occurs is slightly different. For example, acousto-optic conversion actually holds the following relationship: That is,

[0057]

(Equation 1)

Here, Δη _eff is the difference between the effective refractive indices in the optical waveguide (ie, birefringence in the waveguide), λ is the wavelength at which conversion occurs, and Λ is the wavelength of the sound wave. Regarding the variation due to the manufacturing process, the actual value of the birefringence is different between the two waveguides where the conversion occurs for the two polarized lights. If the sound waves have the same wavelength (radio frequency), the result is different conversion wavelengths λ _U and λ _L for the two converters, upper U and lower L. The difference Δλ = [λ _U -λ _L ] is nominally 0.2 nm
It is in the size of. The two transformed polarizations are present at the filter output with a slightly shifted spectrum, which results in optical expansion of the filtering curve.

The acousto-optical device 10 shown in FIG. 1 can operate as a band-pass filter. Assuming that electromagnetic radiation at multiple wavelengths is present in the input optical fiber 3,
Radiation of the desired wavelength is selected by the radio frequency generator 6,
It will be present in the output optical fiber 2.

The operation of device 10 operates polarization controller 9 so that radiation traveling along the optical path between first output port 205 and second input port 203 does not cause any change in polarization state. This makes it possible to optimize for a specific wavelength. This optimization can be performed for all wavelengths if non-full rotation is guaranteed by the polarization maintaining elements (fiber and isolator) between ports 205 and 203. In this case, the rotation of polarized light can be avoided by appropriately selecting the connection orientation without using the polarization controller 9.

For example, the acoustic light switch 8 shown in FIG.
The selective phase of the wavelength for the device shown in FIG. For the sake of clarity, the polarization dividers 104 and 105 are provided with optical waveguides 111, 112, 115, 116 and 113, 11
4, 117, 118 is transmitted along an optical waveguide corresponding to cross transmission, and TM
The polarized light is transmitted along the optical waveguide corresponding to the bar transmission (for example, 111 to 11 in the case of the polarization divider 104).
5) Refer to a specific case (for example, in the case of the divider 104, the cross transmission is 111 to 116).

One skilled in the art will appreciate that the principle can be extended to cases where the polarization divider produces the opposite behavior (bar transmission for TE polarization and cross transmission for TM polarization). It will be obvious. When an appropriate selection signal is applied to the electrodes of the piezoelectric transducers 123, 124, the filter is switched to its on state and changes to a cross transmission state (cross state) for the selected wavelength. In such a situation, input port 202
And 203 are connected to cross output ports, ie 205 and 204, respectively. For such a purpose, the transducers 123, 124 are driven at a driving frequency f _RF
(About 170 for devices operating at about 1550 nm)
A surface acoustic wave is generated at a radio frequency of 210 ± 10 MHz for a device operating at ± 10 MHz, about 1330 nm. The drive frequency corresponds to the resonance light wavelength at which the polarization conversion TE-TM or TM-TE required to be selected occurs.

The light radiation present in the input optical fiber 3 enters the input port 202 and reaches the polarization divider 104, at which time the polarization components TE and TM are separated and proceed to the optical waveguide branches 116 and 115, respectively. TE component and TM
The components propagate along the optical fibers 120, 119 to the acousto-optical conversion stage, respectively. The polarization component having a wavelength different from that required as the output wavelength (selected by the radio frequency generator 6) is transmitted to the optical waveguide branch 12 of the conversion stage 108.
0, 119 are passed unchanged and then sent again to the polarization divider 105 to be synthesized.

The radiation thus recombined is sent in waveguide 106 out of port 203 in an invariant manner. The optical isolator 7 absorbs the radiation having an unselected wavelength transmitted from the port 203. However, the TE polarization component present at the first input port 202 at the predetermined wavelength selected by the radio frequency generator 6 is converted along the optical waveguide 120 to the orthogonal TM polarization state. During this conversion, the TE electromagnetic wave propagates in a direction opposite to the direction of the sound wave generated by the electroacoustic transducer 124,
, A frequency shift Δf which is a negative sign is generated.

[0065]

(Equation 2)

Here, γ _L is the center frequency of the conversion peak for the lower converter L corresponding to the wavelength λ _L defined above,
γ _L ′ is the center frequency of the peak of the converted light at the output of the lower converter L, and f _RF is the transducer 123, 12
4 is the driving frequency.

The TM radiation resulting from the conversion is sent by polarization divider 105 to optical waveguide 118 (bar transmission),
Next, the first through the optical waveguide 107 of the acousto-optical switch 8
To the output port 205. This radiation passes through the optical isolator 7 and the polarization controller 9 to produce a zero rotation of the total polarization of zero. The TM radiation is re-introduced into the same acousto-optical switch 8 via the second input port 203. The radiation containing TM polarization is sent by the optical fiber 106 inside the switch 8 to the polarization divider 105, which transmits the radiation from the optical waveguide 117 to the input waveguide 114. The radiation containing the TM polarization travels along optical fiber 119 in a direction opposite to the direction of the sound waves generated by electroacoustic transducer 123.

In the acousto-optic interaction, TM-T
E conversion occurs at the conversion wavelength (λ _U ) of the upper converter U,
A frequency shift occurs with the value of: That is,

[0069]

(Equation 3)

Here, ν _U is the center frequency of the conversion peak for the upper converter U corresponding to the wavelength λ _U , and ν ′ _U is the center frequency of the converted light peak at the output of the upper converter U.

This light produces a first conversion centered on wavelength λ _L and a second conversion centered on wavelength λ _U. This light thus has two conversion functions for the two conversions:
That is, conversion is performed using a conversion function equal to the product of the lower U and the upper L. This conversion function is the wavelength λ _m between the two wavelengths λ _L and λ _U
Has a center wavelength of Applicants have also observed that the total frequency shift Δf _tot is zero. That is,

[0072]

(Equation 4)

Similar considerations are possible for TM radiation entering the acousto-optical switch 8 shown in FIG. 1 via the input port 202. Therefore, the total frequency shift Δf _tot becomes zero again. Therefore, in the acousto-optic bandpass filter shown in FIG. 1, the frequency shift is null.
It is.

Further, since the TM light again encounters the conversion by both the filtering functions of the two converters of the upper U and the lower L, the total conversion center wavelength becomes λ _m again. That is, for the device shown in FIG. 1, there is no conversion wavelength difference between the two polarizations, and thus there is no expansion of the overall emission spectrum.

In the embodiment as shown in FIG. 1, light passes through the acousto-optical switch 8 twice (double-pas
Perform s). Such a configuration presents all the advantages of a two-stage configuration. In particular, the filtering performance of switch 8 is doubled. Starting with a switch 8 having an extinction ratio of 20 dB and side lobe suppression of 20 dB, a single pass transfer function (si
ngle-pass transfer funti
ON), a filter having an extinction ratio of 30 dB or more and a side lobe suppression of 30 dB or more can be obtained due to two passes of the optical signal through ON). The passband spectrum is 1
It is reduced to 2/3 of the pass band width.

European Patent Application 0814364 A
For conventional two-stage devices with a common substrate, such as No. 1, Applicants have experimented by individually varying the temperature of the stages and providing individual frequency ratios for the stages to achieve matching between the stages. . In an attempt to achieve optimal thermal conditions for the best performance of the entire filter or switch, Applicants individually controlled the temperature of the two stages with an accuracy of about 0.1 ° C. Furthermore, another possible approach is to drive each stage at a respective frequency ratio. 1st stage and 2nd stage
If there is a mismatch between the stages, the two radio frequencies must be individually calibrated to ensure optimal overlap between the two stages of the filtering curve.

In contrast, the arrangement of FIG. 1 of the present invention has the advantage that by making two passes of the same filter, the matching of the characteristics of the first and second stages is automatically guaranteed. Applicants have observed that in the configuration of FIG. 1, only one temperature stabilization is required, and there is no need to maintain the two temperatures in alignment. Furthermore, one radio frequency may be used, and the first and second stage filtering curves are always fully automatically matched. Thus, the device of FIG. 1 is much easier to optimize than using a conventional device.

If the polarization rotation between ports 205 and 203 is equal to 90 ° instead of zero, compensation for frequency shift is still provided, but wavelength shift of the converted polarization is not compensated. Indeed, each polarization makes two passes through the same converter, presenting a slightly shifted filtering curve compared to the orthogonal polarization filtering curve. The result is that the overall bandwidth varies slightly.

When the polarization control is not performed, the light is transmitted to port 2
Some rotation between 05 and 203 may occur. Thus, compensation for wavelength shifts is not needed, but compensation for frequency shifts is still needed. The result is moderate performance between the best case (0 °) and the worst case (90 °).

With the polarization controller 9, zero rotation can be achieved, and thus compensation for wavelength shifts for certain wavelength values. However, such a change causes dispersion in the fiber to change the rotation of the polarization, compromising wavelength shift compensation. Port 2 with fiber and optical isolator with zero polarization rotation maintaining polarization
If fixed between 05 and 203, what would be perfect for any wavelength is provided.

In summary, at least three embodiments of the device corresponding to a good improvement are possible:
(1) a device that performs frequency-shift compensation independent of wavelength and does not perform polarization control between ports 205 and 203;
(2) Perform frequency-independent frequency-shift compensation and fixed wavelength-to-wavelength shift compensation (the device 1 shown in FIG. 1).
0), a device for controlling the polarization between the ports 205 and 203, and (3) a device including a polarization maintaining element between the ports 205 and 203 for compensating the frequency shift and the wavelength shift independent of the wavelength.

Applicants have noted that wavelength shift compensation is not required in certain situations, such as when switch 8 exhibits negligible birefringence non-uniformity or notch depth is not large. In these cases, the configuration without polarization control provides an acceptable solution to the compensation of the frequency shift.

European Patent Application 08143364 A
In the first two-stage device, two filters are integrated on the same chip. According to the embodiment of FIG. 1 of the present invention, only one filter is integrated on the chip, and more available space is obtained. Thus, the first stage of the configuration of FIG. 1 can be achieved with higher performance than the single stage of the previously described two-stage device of the European patent application, improving the overall filter performance.

In the device of FIG. 1, while the passage of light between the first and second stages is developed by means of fiber connections,
In a two-stage single-chip device for the above applications, direct passage through the waveguide on the chip occurs. The loss of switch 8 can be reduced by optimizing the efficiency in the pigtail process for the waveguide-to-fiber connection of FIG. 1 or by using planar or micro-optics technology for the connection.

The acousto-optical device 10 of FIG. 1 is suitable for operating as a band elimination filter by reversing the direction of the optical isolator 7. In this case, the unconverted light is not blocked by the isolator, which in turn blocks the converted light coming from output port 205. As a result, unconverted light is present at output port 204 after two passes through the device. Such a configuration acts as a notch filter.

The device can operate without polarization control, as described above. Furthermore, even if the polarization controller 9 is inserted and the polarization rotation between the ports 205 and 203 is set to zero, the light of each polarization section will pass through both converters. The result is that there is no difference between the wavelengths of conversion for the TE and TM modes, resulting in a completely overlapping notch curve.

An optimization must be made for each value of the selected wavelength. Furthermore, according to the polarization maintaining element,
This effect is maintained for all wavelengths. clearly,
Since the observed light is unconverted light, there is no frequency shift problem. Such a configuration for a band reject filter offers the same advantages as a band pass filter. Filtering performance can be doubled and notch depth is 20dB
In the case of the switch 8, a two-stage filter having a notch depth of more than 30 dB can be achieved.

To help achieve maximum notch depth for non-polarized light in the TE or TM modes, Applicants use the configuration of FIG. 1 (with proper polarization control between ports 203 and 205) and two It has been observed that compensation for wavelength shifts between polarizations is advantageous. In addition, applicant has experimentally found that the device described in European Patent Application No. 08114364 A1 exhibits notch cancellation equal to about 32 dB. On the other hand, the acousto-optical device 10 of FIG.
The notch and filter characteristics of FIG.
The above notch depth is exhibited.

The improved performance of the device 10 of FIG.
Possibly a single-stage switch 8 (eg, PIROAS
150-X) is achieved with two passes resulting in better performance than the single stage of the device described in the applications described above. In fact, a two-pass acousto-optic device made of a single substrate with potential problems in the manufacturing process results in poorer performance than achieved in a single-stage device.

The rotation values for each selected wavelength insert the position of the polarization controller 9 of the type described above until a minimum pass bandwidth or maximum notch depth is obtained in a suitable optical spectrum analyzer. The adjustment can be independently optimized. The adjustment step by the polarization rotation includes the fiber maintaining the polarization in the optical path from the port 205 to the port 203 and the (isolator 7).
Can be avoided by the use of devices). In this case, the total rotation of the polarization can be controlled without the need for the polarization controller 9 by appropriately setting the orientation of the fiber at the connection to the ports 205 and 203.

Further embodiments will be described with reference to the drawings, in which the same numbering is used to indicate components of the type already described. These embodiments include an optical circulator that is a light passing device having sequentially (in this case three or four) ports in the ordinate as shown in the following figures by arrows. In general, radiation entering one of the ports is directed to only one of the other ports, and in particular, to the next port according to a pre-established direction. Nevertheless, for the last port, radiation entering according to the sequence is not sent to subsequent ports. Both types of circulators are called open circulators.

Instead, a so-called closed circulator allows transmission in a pre-established direction between each port. The closed circulator can also be used by properly inserting the isolator when required. In the following drawings, an arrow indicates a fixed direction, and connections are made only between ports capable of transmission.

An optical circulator suitable for use in the following embodiment is, for example, an E-tek model PIF
C2TF341000 (4-port circulator),
Alternatively, it is a model PIFC2PR504000 (3-port circulator). However, significant devices will be apparent to those skilled in the art.

In each of the following figures, each connecting ring on the back of the device shows the performance, ie the polarization rotation value that must be given to the ring in order to enable an optimization of the compensation of the wavelength shift. If the polarization rotation value is not specified, no polarization control is intended, and only frequency shift compensation is performed. With the indicated polarization control, the wavelength shift is also compensated for one wavelength at each time point. With a polarization maintaining element, perfect compensation is guaranteed for all wavelengths.

FIG. 5 shows an acousto-optical device 50 according to another embodiment of the present invention. The acousto-optic device 50 includes an input optical fiber 3 connected to a first input port 202 of the acousto-optic switch 8 operated by the radio frequency generator 6. The first output port 205 of the acousto-optical switch 8 is
It is optically connected to the port 20 of the three-port optical circulator 11 via the polarization controller 9 (if necessary). The optical circulator 11 has a port 30 connected to the fiber 12 and a port 10 connected to the second input port 203 of the acousto-optical switch 8. The port 204 of the switch 8 is connected to the fiber 2.

The device 50 of FIG. 5 is suitable to operate as a drop fiber for transmitting a selected wavelength from the device. Radiation of a wavelength selected between radiation entering the device via fiber 3 is applied to first output port 20 of switch 8 operated by radio frequency generator 6.
5 and thereby to the port 20 of the optical circulator 11. Radiation of the selected wavelength from optical circulator 11 is sent to fiber 12 via port 30. Therefore, the device 50 of FIG.
And between 12 and a single pass drop fiber.

Radiation having a wavelength different from the selected wavelength is not subjected to polarization conversion in the acousto-optical switch 8 and is sent from the first input port 202 to the port 10 of the optical circulator 11 via the second output port 203. . From the port 20 of the optical circulator 11, this radiation is reinserted into the acousto-optical switch 8, whereby it is sent to the fiber 2. Thereby, the radiation corresponding to the notch output in the fiber 2 causes two passes in the acousto-optical switch 8.

FIG. 6 shows that the first output port 205 of the acousto-optical switch 8 is optically connected to the port 10 of the optical circulator 11 via the polarization controller 9 (if necessary). The optical circulator 11 has a port 30 connected to the fiber 12 and a port 20 connected to the second input port 203 of the acousto-optical switch 8.

The operation of drop filter 52 of FIG. 6 is similar to the operation described with respect to device 50 of FIG.
For the device 52 of FIG. 6, radiation corresponding to the notch output in the fiber 12 provides a single pass in the acousto-optical switch 8. Instead, the selected wavelength corresponding to the drop output in fiber 2 causes two passes through acousto-optical switch 8.

In these last forms, device 50 of FIG. 5 and device 52 of FIG. 6 are improved versions of the form of device 10 shown in FIG. In fact, these devices operate in the same way, embodying a notch or bandpass filter almost twice the performance of a two-stage integrated device. Compared to device 10, notch filter 50 of FIG. 5 does not emit light in the stopped bandwidth, but makes it available at the fiber output on fiber 12 after a single pass through switch 8. In the same manner, the bandpass filter 52 of FIG. 6 does not emit light outside the passband, but makes it available at the fiber output 12. Thus, devices 50 and 52 provide a drop filter, a device in which both the pass spectrum and the extraneous light are split into different outputs.

In the embodiments of FIGS. 6 and 7, only one of the two outputs is output after two passes through the acousto-optic device, and the other is output after one pass. Because of this configuration, FIG. 5 provides a two-pass notch filter 50 with one-pass drop recovery, and FIG. 6 provides a two-pass bandpass filter 52 with one-pass notch restoration. In FIG. 5, the dropped light emerging after a single pass through device 8 exhibits an equal frequency shift in the two polarizations. Both FIG. 5 and FIG. 6 exhibit a wavelength shift in which the single pass output is not compensated.

FIG. 7 shows another embodiment of the present invention that acts as an add / drop filter. The acousto-optical device 54 of FIG. 7 a includes an input optical fiber 3 connected to a first input port 202 of the acousto-optical switch 8 operated by the radio frequency generator 6. The first output port 205 of the acousto-optical switch 8 is optically connected via the polarization controller 9 to the port 20 'of the three-port optical circulator 11'. The optical circulator 11 'is a fiber 1
2 and a port 10 ′ connected to the port 30 of the optical circulator 11. The port 10 is connected to the fiber 13, while the port 20 of the optical circulator 11 is connected to the second input port 203 of the acousto-optical switch 8. Port 2 of switch 8
04 is connected to the fiber 2. Optical circulator 1
1 'and 11 are of the same form as described above.

The radiation of a wavelength selected from the radiation entering the device via the fiber 3 is sent to the first output port 205 of the switch 8. The selected wavelength is then sent from port 20 'of optical circulator 11' to drop output on fiber 12 via circulator 11 '. Thus, device 54 of FIG. 7a implements a single pass drop filter between fibers 3 and 12.

Radiation having a wavelength different from the selected wavelength is not subjected to polarization conversion in the acousto-optical switch 8 and is transmitted from the first input port 202 to the port 20 of the optical circulator 11 via the second output port 203. You. From port 30 of optical circulator 11, this radiation is applied to port 1
0 'and then re-inserted from port 20' into the acousto-optic switch 8 and thereby to the fiber 2.
Thus, the radiation corresponding to the notch output in the fiber 2 makes two passes through the acousto-optical switch 8.
As mentioned earlier, the polarization controller 9 is not needed.

As a result of one pass through the acousto-optical switch 8, the radiation added from the fiber 13 is sent to the fiber 2. An optical isolator 14 is needed to absorb the radiated portion guided into fiber 13 which is not deflected into fiber 2. This type of radiation is transmitted through fiber 13
Can be light outside the selected conversion bandwidth of the acousto-optic device that is always driven to port 202 when injected through, or the RF is turned off.
ff) may be light added by itself when performed.

If any of the following cases occur,
The optical isolator 14 is unnecessary. That is, 1) no light exists outside the bandwidth added in the input fiber 13,
Light with no added bandwidth will be turned on (tu)
present only when rned on) or 2) the presence of light exiting the device in the input optical fiber 3 is not a problem in the device connected to the filter via this fiber. The added and dropped light exhibits uncompensated equal frequency shifts and uncompensated wavelength shifts resulting from a single pass through switch 8.

FIG. 7 shows a closed three-port optical circulator 11 and an open three-port optical circulator 11 ″.
And alternative forms including: Optical isolators
No components other than the isolator 14 are used.

FIG. 8 shows another acousto-optical device suitable for acting as an add / drop filter. The acousto-optical device 56 includes an input optical fiber 3 connected to the first input port 202 of the acousto-optical switch 8 operated by the radio frequency generator 6 via the isolator 14 or similar device. The first output port 205 of the acousto-optical switch 8 is a four-port optical circulator 1
5 is optically connected to the port 30. The optical circulator 15 has a port 40 connected to the fiber 16,
A port 20 which is connected to a second input port 203 of the acousto-optical switch 8 via a polarization controller 9 (optionally used). Switch 204 port 204
Are connected to the fiber 2.

The radiation can be directed via fiber 3 to device 56 of FIG. 8 and as a result of a single pass through acousto-optical switch 8, light of the selected wavelength is sent to the drop output port corresponding to fiber 16. Can be Radiation at a wavelength different from the selected wavelength causes two passes through the acousto-optical switch 8 and is transmitted to the notch output port 2. The additional radiation is directed to the fiber 12 connected to the optical circulator 15, and the radiation that passes through the acousto-optical switch 8 only once is transmitted to the fiber 2. The optical isolator 14 has the same effective range as the isolator of FIG. 7 so that the same considerations can be made. Even in such a configuration, the additional light and drop light resulting from the single pass exhibit uncompensated frequency and wavelength shifts.

FIG. 9 shows a drop filter 58 that performs double filtering of light selected by passing through the first acousto-optical switch 8 and the second acousto-optical switch 8 '. The second acousto-optical switch 8 'has ports 202', 203 ', 204' and 205 'as shown in FIG. The acoustic light switches 8 and 8 '
They are of the same type as described above, and are preferably made on the same substrate as the previously described PIRAOS type. The radiation of a wavelength selected from the fiber 3 is sent by the optical circulator 11 to the port 203 'of the second acousto-optical switch 8' and then to the filter 17 via the port 204 '. Port 204 of first acousto-optical switch 8 is the notch port of the device described above with respect to FIG.

Next, in a preferred embodiment, the first
And the second optical switch is made on a different substrate. In such an embodiment, a two-pass notch filter is provided that performs a two-pass drop of the erased bandwidth. Thus, this embodiment embodies a notch-drop filter in which both notch and drop performance are doubled with respect to the initial switch 8.
The configuration for this notch filter is the same as in FIGS. 1 and 5, with all the advantages of frequency shift compensation and wavelength shift compensation, if proper polarization control is ensured.

Furthermore, for the device 58 of FIG. 9, the notch output is 2
It is confirmed that a round pass is performed. In the case of a drop filter, a second pass is made as in FIGS. 1 and 6, but with a "twin" filter being integrated on the same chip by the other filter rather than by the same filter. Is desirable. With respect to the drop characteristics of device 58, certain wavelength shifts are left uncompensated due to technical variations in birefringence in the second device of the first, but frequency shifts are compensated again. However, these variations are much reduced than in the previously described two-stage device. The two stages for dropping are integrated with very little lateral spacing so that technical variations can be kept below undesirable values. In a conventional two-stage acousto-optic device on a single chip, the two filter stages are sequentially arranged in cascade on the same chip, ie at a much greater distance along the chip than in the configuration of FIG. . Such conventional designs include greater technical variability than the inventive layout of FIG. Thus, the resulting wavelength deviation for the drop for the two polarizations is much less in the FIG. 9 configuration than in the conventional two-stage configuration. The integration of multiple acousto-optic filters on a single chip as in FIG. 9 minimizes technical problems.

For the connection from the port 30 of the circulator to the port 203 'of the switch 8', no optimum polarization rotation is shown, but in practice two passes through different switches 8 'occur and the compensation by the polarization rotation does not. Invalid. Thus, such a configuration provides wavelength shift compensation for the two-pass notch and frequency shift compensation for the two-pass drop, but provides some wavelength shift between the polarizations.

It can be seen that it is more convenient to make the wavelength shift not on the notch curve but on the down curve so that the notch depth can be maximized. This is the reason for the setup of the configuration in FIG. However, FIG.
6 by connecting the ports of the circulator in FIG. 6 to be connected in the circulator in FIG. 6 and by connecting port 205 'instead of port 203' of switch 8 '. Can be obtained.

FIG. 10 shows an additional filter 60 having a configuration similar to the device 58 of FIG. In FIG. 10, fiber 3 is the additional input port, fiber 17 is the notch output and the output port of the added radiation, and fiber 2 is the input port of the radiation to be filtered. The additional radiation in fiber 2 undergoes a pass through switch 8 ', and thus through switch 8, while the radiation corresponding to the notch output makes two passes through switch 8.

Such a configuration provides a notch-add filter with both a two-pass notch and an additional filter. In the embodiment of FIG. 9, the addition performs a certain degree of wavelength shift between the polarized lights while performing the total compensation for the frequency shift, while the notch performs the full compensation for the wavelength shift. However, the remaining shift is reduced as compared with the conventional two-stage (cascade) configuration.

FIG. 11 shows an addition / drop filter 6 that allows two passes of various outputs for filtering.
2 is shown. Such an add / drop filter preferably comprises three acousto-optic filters 8, made on the same substrate,
8 ', 8 "and one 4-port optical circulator 22. The acousto-optical switch 8" includes ports 202 ", 204" (connected to fiber 21), (connected to optical circulator 22). Ports 203 ″ and 20
5 ".

In FIG. 11, fiber 3 is the additional input, fiber 19 is the input for the radiation to be filtered, fiber 21 is the drop output, and fiber 17 is the output.
Are notches and additional outputs. Such a form represents a combination of the forms in FIGS. 9 and 10. In this embodiment, a two-stage notch, drop and additional filter (add filter) is added to all two-stage add / drop filters.
r). All three functions can be performed with substantially doubled filtering performance compared to the integrated acousto-optic switch 8. As in the previous two configurations, addition and drop are full compensation for frequency shifts, while wavelength shifts are partial, but notches provide full compensation for wavelength shifts. However, the wavelength shift is 2
It is reduced as compared with the wavelength of the additional / drop filter of the stage.

Obviously, the two-stage addition / drop shown in FIG. 11 as compared to the conventional two-stage acousto-optic device is compared to individually controlling and matching the two stages (temperature and radio frequency). And retains all the advantages arising from one stage of control.
These advantages have been described above.

Referring to FIG. 1, wavelength and frequency shifts
There are alternative ways to achieve compensation. This is the band
Refers to a pass filter configuration. Port 205 and Port 2
03 is rotated by 90 °, two cores
Sound frequency f at inverters U and L_UAnd f_LBut
Even if different, frequency shift compensation is achieved. example
For the polarization TE emerging from port 202,
The conversion to TM in the inverter L causes the frequency shift
G: Δf₁= -F_LIs generated. Port 205 to Port 2
03, the polarization is again changed to TE (shift Δf).₁)When
And the light is again converted in the lower converter L for the second time
Receive. The frequency shift at this time is: Δf _Two= + F_L. Subordinate
Thus, the total frequency shift is zero. That is,

[0121]

(Equation 5)

Thus, the conversion wavelength difference between conversions can be compensated by driving each one at its particular radio frequency to obtain the same conversion wavelength as the other. The conversion thus occurs at the same wavelength for the two polarizations. The method of operating the device requires two different radio frequency controls, but allows for both frequency shift and wavelength shift compensation. Polarization is applied to ports 205 and 2
03 is controlled. The same principles discussed in FIG. 1 can easily be applied to the configurations of FIGS. 5 to 11 as will be readily apparent to those skilled in the art.

FIGS. 13 to 17 show still another embodiment of the present invention including the single-converter acousto-optical device 23. FIG. These embodiments are suitable for achieving all the advantages of two-pass filtering and frequency shift compensation. Compensation for wavelength shifts due to birefringence inhomogeneities is then possible only for the notch output.

A device 23 suitable for use in the configuration of FIGS. 13-17 is described, for example, in US Pat.
2,349 or European Patent Application 0805
372 (U.S. Patent No. 08 / 744,792). The single-converter acousto-optical device 23 shown schematically in FIG. 12 has only one electro-acoustic transducer 216 operated by one radio-frequency generator 6 and two acoustic absorbers 221 (arrows indicate the direction of sound propagation). Shown). The device includes input ports 212, 213 and output ports 214, 215.
Further, the device includes optical waveguides 217, 218 and polarization splitters 219, 220.

The single converter (single co
The nvter acousto-optic device 23 has certain advantages over dual converter devices. In the case of a single converter, two waveguides passing through the same acoustic waveguide can be placed very close to each other, thus reducing the length of the polarization splitter and increasing the size of the chip relative to the converter. Parts can be used. The length of the converter can be increased, fixed to the chip length, and the fiber bandwidth can be correspondingly reduced. Instead, using two converters for the two polarizations forces longer spacing between optical waveguides, each located in a different acoustic waveguide. The length of the polarization splitter section increases and is fixed by the chip size, which reduces the allowable length of the converter.

Thus, in a single converter filter, the converter can be lengthened, and thus have a smaller bandwidth as compared to a multiple converter filter. On the other hand, in the case of a single converter, equalization of the filter frequency shift can be achieved only by two passes instead of one pass by using two different stages or the same stage. The operation of the device 23 is similar to the device described for the acousto-optical device 8, but the frequency shift occurs differently because the sound radiation has only one propagation direction.

Further, the TE wave of the selected wavelength entering device 23 is transmitted from polarization beam splitter 219 to waveguide 218, where the interaction between the acoustic light is polarization conversion and frequency shift equal to f _RF. Is generated. TM waves of the same wavelength are transmitted from the polarizing beam splitter 219 to the waveguide 217, where the interaction between the acoustic light produces a polarization conversion and a frequency shift equal to -f _RF . Thus, the frequency shift between the two polarizations is non-zero after one pass, especially at 2 f _RF, which is twice the frequency of the sound wave.

FIG. 13 shows a bandpass filter 64 including a single converter acousto-optical device 23. The input port 215 of the device is connected to the input port 213 via the polarization controller 9 'and the optical isolator 7. The polarization controller 9 'is adjusted to obtain a 90 degree polarization rotation in the light path between ports 215 and 213. Such a configuration makes two passes of the light entering the fiber 3, which allows for compensation of the frequency acousto-optic shift. In this case, polarization control is added to obtain frequency shift compensation. For reasonable compensation for any wavelength, the polarization maintaining element is
5 and 213.

FIG. 14 shows an embodiment 66 for implementing a two-pass notch filter between fibers 3 and 2. In this case, the polarization controller 9 realizes that there is no polarization rotation in the corresponding optical path. in this case,
There is no frequency shift problem (notch light is not converted). Therefore, the polarization is controlled so as to perform the possible wavelength shift compensation in this case.

FIG. 15 shows a two-pass drop and notch filter 68 and a three-port optical circulator 11 which are preferably made on the same substrate by using two acousto-optic filters 23 and 23 '. . Polarization controllers 9 and 9 'provide 0 ° and 90 ° polarization rotation, respectively, for the corresponding optical paths.
In the device 68, the fiber 3 connected to the port 212 is the input port of the device, and the port 21
The fiber 2 connected to 4 has a notch output, and the filter 17 connected to the port 214 'of the acousto-optic device 23' has a drop output. Such a configuration provides wavelength shift compensation for the notch output and frequency shift compensation for the drop output.

FIG. 16 shows a two-pass add / notch filter 70 similar to device 68 of FIG. In particular, fiber 3 connected to port 212 is an additional input, fiber 2 connected to port 212 'is an input port, and fiber 17 connected to acousto-optic device 23' is a notch and additional output. As in FIG. 15, device 70 provides wavelength shift compensation for the notch output and frequency shift compensation for the additional output.

FIG. 17 shows a double pass add / drop filter 72 by using three acousto-optic filters 23, 23 ', 23 "and a four-port optical circulator 22, which are preferably made on the same substrate. ing.
Polarization controllers 9, 9 'and 9 "provide 0 ° and 90 ° polarization rotation, respectively, for the corresponding optical path. In FIG. 17, fiber 3 is the additional input and fiber 19 is the input of the radiation to be filtered. Yes, fiber 21 is the drop output, and fiber 17 is the notch and add power, device 72 of Figure 17 provides wavelength shift compensation for the notch output and frequency shift compensation for the add and drop outputs.

With the polarization maintaining element, the polarization controller provides a means for adjusting the polarization rotation and achieving compensation for each operating wavelength. The polarization maintaining element of the ring at the back of the filter allows reasonable compensation for each wavelength once the proper polarization rotation has been set. Even without polarization control, that is, without frequency shift compensation,
This configuration is still a good solution for two passes in the same device if frequency shift is not a problem.

In the previously described configuration based on a device with a single converter, the converter can be made longer than in a configuration based on two separate converters. Thus, such a configuration can reduce the bandwidth of the filtering curve as compared to a configuration having two converters. Applicants have shown that one or more 0 ° can be used for the devices shown in FIGS. 15, 16 and 17.
It has been observed that certain changes are possible by replacing the polarization rotation of with a 90 ° polarization rotation and inverting the propagation direction of the corresponding number of sound waves. For example, using an acousto-optic stage 23 'having a transducer at a position to reverse the direction of sound propagation as compared to that shown in FIG. 15 and adjusting the polarization controller 9' to obtain a 0 degree polarization rotation. Thus, a modified example of the device 68 in FIG. 15 is obtained. Similarly, for the devices 70 and 72 of FIGS. 16 and 17, one and three alternative embodiments, respectively, can be implemented.

The Applicant has observed that still other forms are possible according to the symmetric properties of the device used, and that those skilled in the art can easily infer from the preceding figures and the preceding description.

[0136] Furthermore, applicants note that the reinsertion of optical signals in the same switch and the connection between the optical switches as described above can be implemented not only by optical fibers but also by micro-optical technology. For example, the re-inserted optical paths are ports 203 and 205 of switch 8 of FIG.
Can be integrated on a semiconductor substrate appropriately connected to the semiconductor substrate.

As described above with respect to the device of FIG. 1, and also for the other configurations shown, a minimum pass bandwidth or maximum notch depth is achieved in a suitable "optical spectrum analyzer". By adjusting the position of the polarization controller up to, rotation values for each selected wavelength can be independently optimized. Furthermore, such adjustment steps can be avoided by using fibers and devices (such as isolators and optical circulators) for the corresponding optical path by maintaining polarization. In this case, the total polarization rotation can be controlled by appropriately setting the fiber orientation with respect to the acousto-optic filter port without the need for a polarization controller. In this way, the set value of the rotation is maintained without the need for reoptimization, even for changes in the selected wavelength.

The advantages provided by the present invention include: (1) Simplification of the form shown by a single temperature control (no temperature difference between the two stages), single radio frequency control, electro-acoustic transducer for a two-stage device and half of the radio frequency power (2) use with reduced, smaller overall size, narrower bandwidth, and notch output as an add / drop filter;
The ability to remove frequency shifts and wavelength shifts if necessary and implement devices that do not contribute to effective polarization; and (3) the ability to remove frequency shifts even when the radio frequencies in the two converters are different.

Referring to FIG. 18, a radio frequency tunable loop laser generator 74 is described. FIG.
The laser generator 74 shown at 8 includes a doped fiber optical amplifier having a port connected to the optical splitter 24 and another port connected to a portion of the fiber 2.
Optical splitter 24 is connected to output laser fiber 4 and a second portion of fiber 3. A second portion of the fiber 3 is connected to an input of a first polarization controller 5, one of its outputs being optically connected to a first input port 202 of the acousto-optical switch 8. This switch 8
Connected to a radio frequency generator 6. The first output port 205 of the acousto-optical switch 8 is connected to the optical isolator 7,
Its output is optically connected to a second polarization controller 9. The output of the second polarization controller 9 is connected to the second input port 203 of the acousto-optical switch 8.

The second output port 204 from the acousto-optical switch 8 is connected to the first part of the fiber 2 extending to the input of the optical amplifier 1. Suitable erbium (Er) doped fiber optical amplifiers may be, for example, "Amplifox-OP 980F" manufactured by the Applicant or an equivalent apparent to those skilled in the art.

The other components of the laser generator shown in FIG. 18 are, for example, of the same type as described above, and are therefore denoted by the same reference numbers. Optical splitter 24
Has, for example, a splitter ratio of about 20 dB (1% of the power generated by the amplifier is sent to the laser output fiber 4). Splitter ratio values different from -20 dB can be appropriately selected. Optical splitters suitable for use in the device of FIG. 18 are known and, for example, "E
-TEK ".

The optical amplifier used by the applicant in the tested embodiment includes input and output optical isolation elements along the optical path for optical signals. However, Applicants have concluded that a laser according to the present invention can also be operated with an amplifier having a single optical insulator (eg, an input or output optical insulator). Alternatively, the optical isolation element may be coupled along a half ring located external to the optical amplifier and including the optical amplifier.

In the framework of the present application, an optically isolating element is a device that causes a loss in light propagating in the other direction that causes the loss of light to be significantly greater than the loss of light propagating in the opposite direction. In general, the losses in the two directions must be large enough to reach a net loop gain in only one direction.

The operation of the laser schematically shown in FIG. 18 is as follows. The laser shown in FIG. 18 operates by supplying pumping light energy of a wavelength that can excite the fluorescent dopant contained in the doped fiber (active fiber) of the amplifier 1 to a laser emission state. The dopant may be attenuated from this lasing state to the ground state by emission of an optical signal of a predetermined wavelength, that is, simultaneously with or after passage of the optical signal having the same wavelength through the active fiber. This optical signal is passed multiple times through the active fiber of the amplifier 1 due to the configuration of the laser cavity and is therefore amplified multiple times until it reaches a level sufficient to overcome the losses. An optical amplifier other than the fiber amplifier, for example, a semiconductor optical amplifier can be used. As a result, device 74 can generate a laser signal or an emission signal, which can be retrieved at output 4. The emission wavelength is further controlled by an acousto-optical switch 8 which is connected to a radio frequency generator 6 and which interacts with the polarization controllers 5 and 9 to allow the radiation at the required wavelength to be amplified in the laser loop. .

The phase of selecting the laser emission wavelength by the acousto-optical switch 8 shown in FIG. 18 will be described in further detail below. The optical path out of the second output port 204 of the acousto-optical switch 8 and including the fiber 2, the amplifier 1, the fiber 3, and the polarization controller 5 to the first input port 202 is hereinafter referred to as a "left half loop (ref
t-hand half-loop). The optical path out of the first output port 205 of the acousto-optical switch 8 and including the optical isolator 7 and the polarization controller 9 to the second input port 203 is described below as "right-hand half-loop". loo
p) ".

In particular, consider an embodiment of the present invention in which the first polarization controller 5 is controlled such that the left half loop does not cause any rotation of the polarization of the incoming radiation. Similarly, the polarization controller 9 is controlled so that the right half loop also does not rotate. Particular embodiments are referred to as 0 ° -0 ° lasers for simplicity.

The acousto-optic filter can be different from the acousto-optic switch 8, for example the device described with reference to FIG. FIG. 1 similar to FIG. 18, but clearly showing the degree of polarization rotation caused by the two (right and left) half loops
9 is referred to. For clarity, the TM polarization is transmitted via a bar (eg, polarization divider 104).
Are transmitted along the optical waveguide corresponding to (111 to 115), but the polarization dividers 104 and 105 come from any of the optical waveguides 111, 112, 115, 116 and 113, 114, 117, 118. The polarization of the TE wave is cross-transmitted (for example, the divider 10
4, reference is made to the specific case for a switch 8 as detailed in FIG. 2 in which the cross transmission is of the type sent along an optical waveguide corresponding to 111 to 116). One skilled in the art will understand that such a principle is the opposite of what a polarization divider describes below (bar transmission for TE polarization and cross transmission for TM polarization).
It will be clear that this can be extended to the case where

When an appropriate select signal is applied to the electrodes of transducers 123 and 124 in FIG.
Is switched to the ON state, and changes to a cross transmission state (cross state) for the selected wavelength. In such a state, input ports 202 and 203 are connected to cross output ports, ie, ports 205 and 204, respectively. For this purpose, transducers 123 and 1
24 generate two radio frequency surface acoustic waves at a drive frequency f _RF (approximately 174 ± 10 MHz for a device operating at approximately 1550 nm, 210 ± 10 MHz for a device operating at approximately 1300 nm). This frequency corresponds to the resonant light wavelength at which the polarization conversion (TE-TM) or (TM-TE) occurs for the wavelength of radiation for which selection is required.

As mentioned above, even when the transducers are provided with the same radio frequency, the conversion wavelengths in the two converters are slightly different. Actually, in the case of acousto-optic conversion, the relationship according to the above equation (1) is maintained. For manufacturing process variations, the actual value of birefringence is different for the two waveguides where the conversion occurs for the two polarizations. If the sound wavelengths (radio frequencies) are the same, the result is different conversion wavelengths λ _U and λ _L for the two converters, upper U and lower L. Difference Δλ = |
λ _U -λ _L | is usually about 0.2 nm. At that time,
The two converted polarizations are present at the fiber output in a slightly shifted spectrum, which results in optical expansion of the filtering curve. For a laser with a much narrower linewidth, the filter curve of the acousto-optic device will be near its peak, determining the presence of two oscillation wavelengths, one for each polarization shifted by Δλ as indicated above. be able to.

In the above-described embodiment, the emitted light passes through the left half-loop without causing a net change in polarization, and then enters the first input port 202 where the polarization divider 1
Reach 04. At this point, the polarization components of TE and TM are split, and optical waveguide branches 116 and 15 respectively.
And then the optical fibers 120 and 11 respectively.
9 and propagated to the acousto-optic conversion stage.

A polarization component having a wavelength different from that required to be the emission wavelength of the laser (selected by the radio frequency generator 6) passes through the branches 120 and 119 of the conversion stage 108 unchanged. These components are then
These are sent to the polarization divider 105 to be recombined.
The radiation thus recombined is sent in waveguide 106 so as to exit port 203 in an unaltered form. An optical isolator 7 absorbs the radiation with a non-selected wavelength coming from port 203.

However, the TE polarized light component present at the first input port 202 at the predetermined wavelength selected by the radio frequency generator 6 is orthogonal to the TM along the optical waveguide 120.
Converted to polarization state. During this conversion, the TE electromagnetic radiation is propagated in a direction opposite to the direction of the sound waves generated by the electro-acoustic transducer 124, and thus, as shown in Table 1, this electromagnetic radiation has a negative sign Δf _L = −f _RF This causes a frequency shift of the light.

The resulting TM radiation from the converter is sent by the polarization divider 105 to the optical waveguide 118 (bar transmission) and then to the first output port 205 of the acousto-optical switch 8 via the optical waveguide 107. First, consider the optimized state. The radiation passes through the optical isolator 7 and the polarization controller 9 without causing a net change in its polarization state. This TM radiation is
It is re-introduced into the acousto-optical switch 8 via the second input port 203. This radiation, including TM polarized light,
06 to the polarization divider 105. The polarization divider 105 transfers this radiation from the optical waveguide 117 to the waveguide 11.
4 and propagated from this waveguide along the optical fiber 119 in a direction opposite to the propagation direction of the sound wave generated by the electroacoustic transducer 123. This interaction between acoustic light results in a TM-TE conversion and a positive frequency shift of light with a value of Δf _U = f _RF . In summary,
The TM radiation entering the polarization controller 5 causes a total frequency shift in this passage through the laser loop:

[0154]

(Equation 6)

[0155] And, the radiation including the TE polarized light is present at the second output port 204. From here, the TM radiation again passes through the left half loop, passes through the optical amplifier and polarization controller 5, and is reintroduced into the acousto-optical switch 8 at the input port 202 without causing a net change in polarization.
Thus, during a selected phase of the emission wavelength of the laser, a frequency shift is introduced for every two passes of radiation through the filter.

Still another embodiment of the present invention is shown in FIGS. 20, 21 and 22. According to another embodiment with respect to FIG. 20, the right half loop including polarization controller 9 produces a full rotation of 90 °, while the left half loop including polarization controller 5 produces no rotation with respect to laser device 78. . The embodiment can be easily implemented from 0 ° to 90 °.
° called. The operation of the laser is similar to the operation of the 0 ° to 0 ° laser described above and will be apparent to those skilled in the art.

In such a configuration, when TE-type radiation goes to the first polarization controller 5, with two passes through the laser loop, ie a zero frequency shift after two passes through the acousto-optical switch 8, Second of acousto-optical switch 8
At the output port 204.

FIG. 21 shows a laser configuration 80 of the 90 ° -0 ° type. That is, the left half loop produces a total of 90 ° rotation, and the right half loop produces a 0 ° rotation in polarization. In such a configuration, when TM-type radiation enters the polarization controller 5, two passes through the laser loop with zero frequency shift, ie two passes through the acousto-optical switch 8, will cause At the second output port 204, TM type radiation occurs.

The configurations described in FIGS. 19-21 force the light to alternatively pass through the two converters U and L. Thus, this total filter characteristic is the product of the two filter characteristics shifted by Δλ of the two polarizations, and is therefore the same for the original TE and TM light. These configurations completely compensate for even the converted wavelength shift Δλ between the two polarizations, permit lasing in only one line, and are the same for both TE and TM polarizations.

In a particular embodiment of the device 82 shown schematically in FIG. 22, both the right and left half loops rotate a total of 90 ° in the polarization of the radiation passing through them. Resulting in a 90 ° -90 ° laser configuration. As can be readily seen from FIG. 22 and the foregoing, this again makes it possible to achieve full compensation for the frequency shift after two passes of the laser loop, in other words, two passes of the acousto-optic switch 8. enable.

On the other hand, in the device 82 of FIG. 22, each polarization always passes through the same converter, so that the wavelength shift is not compensated. Applicants have shown in FIGS. 20, 21 and 22
Configuration for the transducers 124 and 123
It has been observed that even when the radio frequencies are different, it is suitable for compensating for the frequency shift due to the interaction between the acoustic lights.

The present invention provides a number of advantages over the prior art. Applicants have observed that compensation for the frequency shifts that occur in the interaction between acoustic light occurs exactly with lasers according to the embodiments of the present invention shown in FIGS. In fact, the passage of light in converters U and L occurs more than once with a directional characteristic that ensures that each of the two polarizations produces the same number of shifts with equal acoustic frequency but opposite sign values.

The devices according to the invention all have the advantage resulting from the two passes indicated above, performed by a filter. By removing the frequency shift, stable laser oscillation is achieved. The sum of the frequency shifts in any step of the optical signal in the laser ring shifts the signal spectrum into the gain region of the amplifier without enabling laser operation.

Applicants have noted that when the components in the left and right half-rings (eg, all fibers, optical amplifier 1, optical isolator 7, optical splitter 24) do not maintain polarization, when the wavelength is changed, We have experimentally found that the rotation does not stop at the optimal state. This is due to the dispersion characteristics of the material of the optical fiber. Changes in the refractive index result in changes in the way the fiber rotates the polarization. This determines the periodic branching of the single-line spectrum in the doublet spectrum when the laser is tuned, as shown below, where the competition between the two polarizations is irregular from the ideal curve of the tuning curve. Causes periodic fluctuations.

Fibers and components that maintain polarization (eg, optical amplifier 1, optical isolator 7, optical splitter 24) should avoid such occurrences. Optical splitters that maintain polarization are available, for example, from E-TEK. The active fiber for maintaining the polarization with respect to the optical amplifier 1 is, for example, Lucent Technology.
available from Ogis. In such a configuration as shown in FIG. 23, there is no need for a polarization controller. The desired polarization rotation in the two half rings is
This can be achieved simply by a proper choice of orientation in the connection of the fiber with the waveguide of the integrated filter.

The acousto-optical switch 8 can be set to select more than one wavelength by simply applying the corresponding set of radio frequencies. In this way, more than one wavelength can be selected in the laser ring, and each selected wavelength can be tuned independently of the other wavelengths. In the proposed configuration, it is possible to select and independently tune many lines for lasing. The effective continuous oscillation of the laser in many lines depends only on the gain characteristics of the optical amplifier. With proper gain behavior in the wavelength range of interest, the laser can act as a multi-line source, where each laser line is tuned independently of the other lines. The acoustic filter can add multiple wavelength oscillation.

The transmission of each selected wavelength can be changed to equalize the net gain for the wavelength by changing its radio frequency power. This equalization should be controlled during tuning of each wavelength.

The applicant constructed a laser according to the disclosed invention and performed an experimental test described below. Experimental test : A laser as shown in FIG.
In this laser, two polarization controllers 5 and 9
It was possible to vary the rotation of the polarization introduced into the two half loops. The test was performed with an acousto-optic filter PIRAO.
Repeated for three samples of S-150X. These samples are called S1, S2, S3. Other devices used were described above. In particular, the optical isolator was such that it caused a 34 dB loss for radiation at wavelengths not selected by switch 8.

The laser loop had the following structural characteristics. That is, the estimated length of the loop of 20 m measured a loss of 15 dB (not including Er loss) in the loop, and the output coupling was 21.6 dB (0.7%). The acousto-optical device was optimized to operate at a temperature of 40 ° C.

As a result of the measurement by the power meter, the following power characteristics of the laser light source were known. That is, laser output power: -10 dBm (0.1 mW); estimated power in the cavity: 11.6 dBm (14.5 mW). The time characteristic is 1
The photodiode for the 5 GHz band was connected to the output fiber 4 and then measured with an electric spectrum analyzer and an oscilloscope. Emission was continuously observed with polarized light. A random amplitude modulation of the output signal could be observed, probably due to instability in the laser loop.

[0171] The spectral properties were evaluated using an optical spectrum analyzer and a scanning Fabry-Perot filter. The laser light source according to the present invention is adapted to jump quickly between mode groups and oscillate in multiple modes. Although the individual line width was 60 MHz, the oscillation band was found to have a width of about 4 GHz.

FIG. 24 shows an optical spectrum analyzer (O
The envelope of the oscillation peak measured in SA) is shown. As can be seen in FIG. 25, the peak wavelength changes over time and oscillates within a range of about 0.15 nm, corresponding to a temporal change in the conversion frequency of about 20 KHz. These can be considered as good stability characteristics.
The stability of the emission peak extends from the second output port 204 to the first input port 202 and to the first output port 20.
It appears to depend on the change in polarization along the light path extending from 5 to the second input port 203. Thus, the stability appears to be related to the state of the two polarization controllers 5 and 9.

Applicants have observed that for certain states of the polarization controllers 5 and 9, the peaks appear as described above (stable state). When the state of the even controller is changed, the shape of the peaks becomes irregular, resulting in two distinct peaks separated by about 0.18 nm (relatively less stability; shown in FIG. 26). By changing the state of the controller, it is possible to regularly change from the one peak state to the two peak state.

Measurements were also made to evaluate the tuning characteristics of the laser light source. A typical tuning curve obtained by changing the radio frequency in the PIRAOS-150X acousto-optical switch 8 is shown in FIG.

FIG. 28 shows a corresponding change in the output power of the laser. The laser can be continuously tuned over the gain band of erbium (ie, 1522 nm to 1565 nm), in other words, over the gain band of the active fiber dopant of amplifier 1, but can be extended to the laser bandwidth. I found it to be. Polarization conversion theory due to the interaction between acoustic light predicts the inverse relationship between frequency and converted wavelength. Applicants have experimentally observed that the change in emission wavelength as a function of sound frequency can indeed be well interpolated from a form curve.

[0176]

(Equation 7)

The values of the constants A and B appear to depend slightly on the acousto-optical device used, and the constants are always approximately the same. Table 2 below shows the acousto-optic filter PIR
Shown are the values found for two constants using three samples of AOS-150X, S1, S2, S3.

[0178]

[Table 2]

The remaining standard deviation indicates the standard deviation (root mean square) of the difference between the measured value and the value in the theoretical curve. The deviation of the experimental data from the interpolated curve shown in FIG. 29 can be attributed to the “non-ideal (non-ideal)
ity) ”.

It can be seen that there was a cumulative loss of peak stability that could be recovered by reoptimizing the state of the polarization controller when the radio frequency changed while the wavelength of the peak was shifted. Was. Attention has been focused on the curve of deviation from the ideal case of the tuning process. This state is shown in FIG. 29 as a function of the emission wavelength.

In FIG. 29, there are two overlapping “disturbances”, a gradual change (about 15 nm) and a fast change (2 nm to 1 nm).
nm) and very irregular tendencies can be distinguished. The gradual change is due to the fact that the gain curve of erbium is not flat but has peaks and small slack. The product of the gain curve and the filtering curve is
Determine the emission wavelength. In the resulting curve, the peak is further shifted with respect to the peak of the filtering curve as the slope of the gain curve increases at the peak (the gain curve peak "pulls" the oscillation peak).
When a passive filter is introduced into the loop, the shape of the gain curve changes and, as expected, the trend of long-term modulation follows the change. Thus, such a component of the deviation from the ideal tuning curve is inherent in the light emitting process, and correcting the shape of the gain curve appears to eliminate this.

Fast changes appear to be associated with changes in the polarization of light in the loop. As mentioned earlier,
The devices of FIGS. 19, 20 and 21 allow no wavelength difference between the two polarizations. Applicants have concluded that these three configurations correspond to the most stable state of the laser with only one emission peak shown in FIG.

In the fourth embodiment shown in FIG.
Each polarization is converted twice in the same converter, so that the filtering curves for TE and TM appear shifted. Such a configuration appears to be associated with a relatively unstable condition having two distinct emission peaks as shown in FIG. Regarding the tuning characteristics, the wavelength shift for a certain polarization form and the presence of the dispersion characteristics of the common fiber are due to the periodic changes shown in the deviation curves in FIG.

When the laser is tuned, ie, the wavelength selected by the acousto-optic device is changed, the rotation of the polarization in the two rings of the laser structure is changed by the change in the index of refraction in the fiber. Therefore,
This configuration changes gradually, causing a wavelength shift between the polarization components, resulting in modulation of the tuning curve. This modulation is due to the incremental hopping of the laser wavelength from one line to the other corresponding to two different polarizations when a shift is induced. The periodicity of the modulation is due to the recovery of the initial polarization state as the polarization rotation deviation approaches 360 °.

As will be readily appreciated, such a periodic modulation can be achieved if the two half rings are designed so that when the wavelength is tuned, they do not change the polarization state of the light globally. Can be avoided. This can be achieved by using all polarization maintaining fibers that do not exhibit polarization effects when the wavelength is changed using optical amplifiers, optical splitters and isolators that both maintain polarization. All of these devices are readily available to those skilled in the art and their configuration can be effectively set up.

The half ring is constituted by polarization maintaining elements, and the lasers are 0 ° -0 °, 0 ° -90 °, 90 ° -0
It is expected to operate in the most stable state in any configuration of °. However, even if only the right half ring is polarization-maintaining, stable operation and fast elimination of tuning curves can be achieved. The left half ring may remain unpolarized if the right half ring produces a full polarization rotation of 0 °. In this case, the lasers are 0 ° -0 ° and 90 °-
Oscillates between 0 ° configurations. As described above, both of these configurations do not cause a wavelength shift, and thus generate a stable light emitting state (one peak).

However, in the configuration of 0 ° -90 °, two passes are required, but in the configuration of 0 ° -0 °, one pass of light in the ring is required for one round (two passes of the switch 8). Times). Therefore, a different number of rounds (ro
The laser shifts between each state due to an under-trip passage. For this reason, it is expected that the embodiment of maintaining polarization in only the right half ring will be slightly less stable than maintaining polarization in both half rings. 90 in the right half ring without polarization maintenance in the left half ring
A polarization rotation of 90 ° is undesirable because it can result in instability with 90 ° -90 ° configurations.

In summary, a further improved embodiment of the present invention provides a number of advantages over conventional configurations and includes: (1) no polarization control in the two half-rings, which provides good tuning performance of the laser but can result in periodic instability and periodic "fast" modulation of the tuning characteristics; (2) tuning curve Periodic "early"
Maintaining polarization only in the right half ring with 0 ° polarization rotation, providing good tuning performance without modulation but not perfect stability, and (3) periodic “fast” modulation of the tuning curve 0 ° -0 ° or 0 ° -9 to provide good tuning performance without high laser stability
Maintaining polarization in both half rings with a 0 ° or 90 ° -0 ° configuration.

Applicants have measured the tuning deviation curve for the laser with the S1 and S3 filters. The behavior of the various acousto-optic filters used appears to fluctuate (in a limited way) only at the amplitude of the maximum deviation and at intervals of the periodicity of the maximum peak in the fast fluctuations. As will be apparent, these differences may be due to different values of the wavelength shift of the light converted in the two polarization conversion zones.

To improve the performance of the laser, several instructions can be followed. First, special active materials or optical flattening filters
By using an ending filter, the gain curve of the laser can be flattened. Second, stability can be improved by reducing discontinuities in fiber-filter and fiber-to-fiber connections to reduce back reflections at the connections. Third, the cavity length can be reduced to allow activation of a relatively small number of longitudinal modes. Also,
Laser parameters such as output coupling and pump power can be optimized to allow for optimal operating conditions for the laser.

It will be apparent to those skilled in the art that various modifications and variations can be made to the system and method of the present invention without departing from the spirit and scope of the invention. For example, the add / drop configurations are intended to be merely illustrative of preferred embodiments and not limiting. The present invention covers modifications and variations of the present invention provided that they come within the scope of the appended claims and their equivalents.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a bandpass filter according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an acousto-optical switch according to an embodiment of the present invention.

FIG. 3 is a graph showing a characteristic curve of an acousto-optic filter of the type shown in FIG.

FIG. 4 is a graph showing a characteristic curve of the device of FIG. 1;

FIG. 5 is a schematic diagram illustrating two rejection filters according to another embodiment of the present invention using an optical circulator.

FIG. 6 is a schematic diagram illustrating two rejection filters according to another embodiment of the present invention using an optical circulator.

FIG. 7 comprises A and B, where A is a schematic diagram illustrating an add / drop filter according to another embodiment of the present invention using two optical circulators, and B is a closed circuit. FIG. 7 is a schematic diagram illustrating an alternative embodiment using a modified optical circulator.

FIG. 8 is a schematic diagram illustrating an add / drop filter according to another embodiment of the present invention using an optical circulator.

FIG. 9 is a schematic diagram illustrating a device according to another embodiment of the present invention using two parallel acoustic light stages and an optical circulator.

FIG. 10 is a schematic diagram showing a device according to another embodiment of the present invention using two parallel acoustic light stages and an optical circulator.

FIG. 11 is a schematic diagram illustrating a device according to another embodiment of the present invention using three parallel acoustic light stages and an optical circulator.

FIG. 12 is a perspective view showing an acousto-optic filter having a single converter.

FIG. 13 is a schematic diagram illustrating a device according to another embodiment of the present invention using an acousto-optic device having a single converter.

FIG. 14 is a schematic diagram illustrating a device according to another embodiment of the present invention using an acousto-optic device having a single converter.

FIG. 15 is a schematic diagram illustrating a device according to another embodiment of the present invention having two parallel acoustic light stages with a single converter.

FIG. 16 is a schematic diagram illustrating a device according to another embodiment of the present invention having two parallel acoustic light stages with a single converter.

FIG. 17 is a schematic diagram illustrating a device according to another embodiment of the present invention using a device having three parallel stages with a single converter.

FIG. 18 is a schematic diagram illustrating a laser according to another embodiment of the present invention.

FIG. 19 is a schematic diagram showing different forms of a laser according to another embodiment of the present invention.

FIG. 20 is a schematic diagram showing different forms of a laser according to another embodiment of the present invention.

FIG. 21 is a schematic diagram showing different forms of a laser according to another embodiment of the present invention.

FIG. 22 is a schematic diagram showing different forms of a laser according to another embodiment of the present invention.

FIG. 23 is a schematic diagram illustrating a laser according to another embodiment of the present invention using a polarization maintaining fiber.

FIG. 24 is a graph showing the emission spectrum of a laser according to the present invention using a PIRAOS-150X acousto-optic filter.

FIG. 25 is a graph showing the temporal change of the emission peak of the laser according to the present invention.

FIG. 26 is a graph showing an emission spectrum of the laser according to the present invention in a relatively unstable state.

FIG. 27 is a graph showing a tuning curve of a laser according to the invention, showing the emission wavelength as a function of the acoustic frequency.

FIG. 28 is a graph showing the total emission power of a laser according to the present invention during tuning.

FIG. 29 is a graph showing the variation of the tuning characteristics of a laser according to the present invention as experimentally shown by a theoretical interpolator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical amplifier 2 Fiber 3 Input optical fiber 4 Laser output fiber 5 First polarization controller 6 Radio frequency generator 7 Optical isolator 8 Acoustic light switch 9 Polarization controller 10 Acoustic light device 11 Optical circulator 12 Fiber 13 Input fiber 14 Optical isolator 15 4 port optical circulator 16 fiber 17 fiber 20 port 22 4 port optical circulator 23 single converter acousto-optic device 24 optical splitter 30 port 50 acousto-optic device 52 drop filter 56 acousto-optic device 58 drop filter 60 additional filter 64 band-pass filter 68 device Reference Signs List 70 Device 72 Two-pass addition / drop filter 74 Loop laser generator 101 Substrate 104 Polarization divider 105 Polarization divider 106 optical fiber 107 optical waveguide 108 conversion stage 109 central optical waveguide 110 central optical waveguide 113 input waveguide 114 input waveguide 115 output waveguide 116 output waveguide 117 optical waveguide 118 optical waveguide 119 optical waveguide branch 120 optical waveguide branch 121 acoustic conduction Waveguide 122 Acoustic waveguide 123 Electroacoustic transducer 124 Piezoelectric transducer 125 Acoustic waveguide 126 Acoustic waveguide 127 Acoustic waveguide 128 Acoustic waveguide 129 Second center cladding 130 Acoustic absorber 202 First input port 203 Second input port 204 Second output port 205 First output port 212 Input port 213 Input port 214 Output port 215 Output port 216 Electroacoustic transducer 217 Optical waveguide 218 Optical waveguide 19 polarizing beam splitter 220 polarizing splitter 221 sound absorber

 ──────────────────────────────────────────────────の Continued on the front page (71) Applicant 591011856 Pirelli Cavies System e. p. A (72) Inventor Salvatore Morasca 22100 Como, Via Milano, Italy 162 (72) Inventor Stephen Schmid Milan, Italy, 20052 Monza, Via Carlo Porta 19/4

Claims (20)

[Claims] An acousto-optic switch including first and second polarization conversion zones coupled together between first, second, third and fourth optical ports; and said second and third optical ports. A multi-pass acousto-optical device comprising: 2. The apparatus according to claim 1, wherein the first and second optical ports are disposed between the first and fourth optical ports and the first and second polarization conversion areas.
The multi-pass acousto-optical device according to claim 1, further comprising a first optical splitter having cross transmission and bar transmission with respect to orthogonal polarization components of the received light, respectively. 3. A cross transmission and bar for orthogonal polarization components of received light, respectively, disposed between the second and third optical ports and the first and second polarization conversion zones. 3. The multi-pass acousto-optical device according to claim 2, further comprising a second optical splitter having a transmission. 4. The multi-pass acousto-optical device according to claim 3, further comprising a polarization controller coupled in series with said optical isolating element. 5. The multi-pass acousto-optical device according to claim 3, wherein the optical combination performs polarization maintenance. 6. An acousto-optical switch within the acousto-optic switch acoustically coupled to the first polarization conversion region and coupled to an RF source.
The multi-pass acousto-optical device according to claim 1, further comprising an upper transducer that generates a first sound wave in the first polarization conversion region having a characteristic frequency determined by the RF source. 7. A characteristic frequency within the acousto-optic switch acoustically coupled to the second polarization conversion region and coupled to the RF source, the characteristic frequency having a propagation direction opposite to a propagation direction of the first sound wave. The multi-pass acousto-optical device according to claim 6, further comprising a lower transducer that generates a second sound wave in the second polarization conversion region having: 8. The first polarization conversion area and the second polarization conversion area are sufficiently close so that the first sound wave travels in both the first and second polarization conversion areas. The multi-pass acousto-optical device according to claim 6, which is arranged. 9. The multi-pass acousto-optical device according to claim 1, wherein the optical insulating element is an optical circulator having at least three ports. 10. The multi-pass acousto-optical device according to claim 9, further comprising another acousto-optical switch coupled to the optical circulator and configured on the same substrate as another acousto-optical switch. 11. An acousto-optical switch including first and second polarization converters coupled between first, second, third, and fourth optical ports; and said first optical port and said fourth optical port. A first optical half ring coupled between the second optical port and the third optical port, the second optical half ring including an optical amplifier; and a second optical half ring coupled between the second optical port and the third optical port and including an optical isolation element. A tunable laser generator comprising: a half ring; and a laser output coupler disposed within one of the first optical half ring and the second optical half ring. 12. The tunable laser generator of claim 11, wherein said first half ring includes an optically insulating element. 13. The first half ring and the second half ring.
13. The tunable laser generator of claim 12, wherein one of said half rings comprises a polarization maintaining element. 14. The first half ring and the second half ring
13. The tunable laser generator of claim 12, wherein one of said half rings includes a polarization controller. 15. The first half ring and the second half ring
13. The tunable laser generator of claim 12, wherein both of said half rings include a polarization controller. 16. The tunable laser generator of claim 12, wherein said optical amplifier comprises an erbium-doped optical fiber amplifier. 17. A method for filtering an optical frequency using an acousto-optical device, comprising: a first polarization conversion region coupled between first, second, third and fourth optical ports; Providing at least one acousto-optical switch including a polarization conversion region; injecting an input optical signal having a plurality of wavelengths into the first optical port; and providing an intermediate optical signal including a subset of a plurality of wavelengths. A method comprising: feeding back from a second optical port to a second optical port via an optical isolation element; and extracting an output optical signal comprising a subset of the plurality of wavelengths from the fourth optical port. 18. A step of dividing an input optical signal into initial components of a TE wave and a TM wave inside the first optical splitter on the switch, and passing the initial component of the TE wave to the first polarization conversion area. Passing the TM wave initial component to the second polarization conversion region; and orthogonally intersecting the TE wave initial component of the input optical signal having the selected frequency to a TM wave intermediate component in the first polarization conversion region. 18. The method of claim 17, further comprising the steps of: phase converting; and quadrature converting the TM wave initial component of the input optical signal having the selected frequency to a TE wave intermediate component in the second polarization conversion region. 19. After the feedback step, splitting the intermediate optical signal into a feedback component of a TE wave and a TM wave within a second optical splitter on a switch; Passing the TM wave feedback component to the second polarization conversion region; and passing the TE wave feedback component having a selected frequency to the TM wave in the first polarization conversion region. 18. The method of claim 17, further comprising: quadrature converting to a final component; and quadrature converting the TM wave feedback component having the selected frequency to a final component of a TE wave in the second polarization conversion region. 20. The method of claim 17, further comprising coupling the input optical signal from the fourth optical port to the first optical port via an optical amplifier.