JP2000235171A - Optical frequency filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an increase of the number of control electrodes even when the number of selectable optical frequencies are increased by serially connecting two optical frequency filter parts, equalizing both channel frequency intervals and differentiating a free spectral range(FSR) only by channel interval. SOLUTION: A first optical frequency selection filter part 1 and a second optical frequency selection filter part 2 are arranged tow stages serially on the same InP semiconductor substrate. The channel frequency intervals of the preceding stage and the poststage are made equal, and the FSRs are made so as to become values different only by channel interval. When a beam is transmitted through two stages of optical frequency selection filter parts, a thing multiplying both transmission characteristics with each other becomes the characteristic of the whole optical frequency filter. By selecting a combination between an optical gate switch transmission-operating of the optical frequency filter part 1 of the preceding stage and the optical gate switch transmission-operating of the optical frequency filter part 2 of the poststage, the number of optical frequencies selectable with the less number of electrodes are remarkably increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical frequency filter. More specifically, the present invention relates to an integrated optical frequency filter used for optical communication, optical switching, optical information processing, optical measurement, and the like.

[0002]

2. Description of the Related Art In order to realize large-capacity communication, wavelength multiplexing optical communication for multiplexing and transmitting optical signals having a plurality of different optical frequencies (wavelengths) on a single optical fiber is being actively developed. Have been done. An optical frequency filter capable of selectively extracting only an optical signal of a specific optical frequency (wavelength) from a multiplexed optical signal composed of a plurality of optical frequencies is an optical add / drop device, an optical cross connect device. It is indispensable for realizing an optical switch.

An optical frequency filter in which a pair of arrayed waveguide grating filters and an optical gate switch are integrated can digitally and quickly select an optical frequency by turning on / off an electrical signal. Promising as a filter suitable for such applications. As a reported example of an optical frequency filter in which a pair of an arrayed waveguide grating filter and a semiconductor amplifier / gate switch are integrated on an InP semiconductor substrate, M. Zirngible et al. (IEEE Photonics.
Technology Letters vol.6, pp.513-515.1994). Here, the operating characteristics of the conventional optical frequency filter will be described with reference to the drawings.

FIG. 3 schematically illustrates a configuration when an optical frequency filter of eight channels is viewed from above.
In this filter, three regions of an arrayed waveguide grating splitter 10, an arrayed waveguide grating multiplexer 11, and a semiconductor amplifier array 12 are formed on an InP substrate 24. The optical waveguide is formed on an InP substrate, and has a structure in which an InGaAsP core layer having a bandgap wavelength of 1050 nm is sandwiched between InP cladding layers.

Therefore, a wavelength of 1550 nm (optical frequency: to
193 THz) and is transparent to light near InP.
The light is confined in the InGaAsP layer having a higher refractive index than that of the above, and the light propagates along the waveguide pattern. Therefore,
When the incident light 25 enters the input waveguide 13 of the first arrayed waveguide grating splitter 10, the incident light 25 is guided to the first slab waveguide 14. In the first slab waveguide 14, since there is no confinement effect by the refractive index in the direction parallel to the substrate 24, the light spreads at a certain angle. The spread light is gradually coupled to the large number of array waveguides 17 and guided to the second slab waveguide 15.

The light converges in the second slab waveguide 15 and couples to one of the plurality of output waveguides 16. Here, a certain difference in length is provided between adjacent waveguides of the arrayed waveguide 17. For this reason,
The position where light is converged in the plane between the second slab waveguide 15 and the output waveguide 16 changes depending on the frequency of the input light. For example, if light of a certain frequency f ₁ is coupled to the first output waveguide 16, light having a frequency different by Δf will be coupled to the adjacent second output waveguide 16.

That is, the first arrayed waveguide grating splitter 10
Functions to distribute light having different optical frequencies to the plurality of output waveguides 16 according to the frequency. On the other hand, the second arrayed waveguide grating multiplexer 11 is arranged such that its input waveguide 18 and output waveguide 21 are arranged in the same manner as the input waveguide 13 and output waveguide 16 of the first arrayed waveguide grating duplexer 10. It is a symmetrical structure with a replacement. Therefore, the second arrayed waveguide grating multiplexer 11 is provided with the first arrayed waveguide grating splitter 1.
In the description of the operation 0, the operation can be understood by considering how light travels in the opposite direction. That is, the second arrayed waveguide grating multiplexer 11 functions to multiplex the demultiplexed light into one waveguide to produce output light 26.

[0008] A semiconductor amplifier array 12 is formed between the output waveguides and the input waveguides of the two arrayed waveguide grating multiplexers and demultiplexers 10 and 11. Semiconductor amplifier
The waveguide of the array 12 has a band gap wavelength of 1550-1.
It is made of 580 nm InGaAsP, and when no current is injected, light near 1550 nm is absorbed,
When current is injected, the light is amplified. That is, the semiconductor amplifier array 12 functions as a gate switch that blocks or transmits light depending on the current ON / OFF. Therefore, by passing a current through a specific semiconductor amplifier to pass light, it is possible to guide only a specific wavelength from the light split by the first arrayed waveguide grating splitter 10 to the output waveguide. Becomes possible.

This optical frequency filter is characterized in that the optical frequency selected electrically can be switched at a high speed. Here, the design of the arrayed waveguide grating filter will be described below. The frequency of light transmitted from a particular output waveguide of the arrayed waveguide grating filter is
It will be periodic. The period of this frequency is called a free spectral range (FSR) and is determined by the following equation. FSR = c / (n · ΔL) (1) where c is the speed of light, n is the effective refractive index of the arrayed waveguide, Δ
L is the difference in length between adjacent arrayed waveguides.

That is, the FSR is determined by setting the length difference ΔL between the arrayed waveguides. Further, the difference in transmission frequency between adjacent output waveguides, that is, the channel frequency interval Δf can be determined by the interval between the output waveguides, the length of the slab waveguide, and the FSR. In the example shown in FIG. 3, the channel frequency interval Δf is 20 GHz, the number of channels (the number of output waveguides) is 8, the FSR is 1.6 THz, and the FSR is exactly the number of channels times the channel frequency interval. Is set to This filter operates as an optical frequency filter that selects only light of an arbitrary frequency from eight multiplexed optical signals arranged at intervals of 200 GHz.

However, in the above prior art, when the number of selectable optical frequencies is increased, an arrayed waveguide grating filter having a large number of output waveguides is required in proportion to the number of optical frequencies. The number also increases. As a result, the size of the element increases, and the number of electrodes increases. Increasing the element size causes a decrease in the number of elements that can be made from a unit wafer and a decrease in yield, and an increase in the number of electrodes leads to an increase in a wiring process for performing electric wiring and an increase in the scale of a control circuit. There was such a problem. As an effective means for increasing the number of selectable optical frequencies, there is a method of connecting optical filters in multiple stages.

FIG. 4 shows a conventional example in which a Mach-Zehnder filter is connected in two stages. Also in the case of the Mach-Zehnder filter, it has a characteristic of transmitting periodically with respect to the optical frequency. The period of the transmission frequency, that is, the FSR is determined by the difference between the lengths of the two branched optical waveguides, and the value is similarly calculated by the equation (1). In this example, the optical waveguide is designed so that the FSR of the first Mach-Zehnder filter 30 is twice the FSR of the second Mach-Zehnder filter 31, and the control electrode 32 is turned on / off.
By performing FF, the optical frequency of the input light 33 is roughly selected by the first Mach-Zehnder filter 30, and the output light 34 is further finely selected by the second Mach-Zehnder filter 31. The number of selectable optical frequencies has been increased.

In the two-stage optical filter described above, the optical filter formed by the array waveguide grating and the optical gate switch described above instead of the Mach-Zehnder filter can be easily replaced. You can analogy. That is, FSR is used as the first filter.
If an array waveguide grating filter having a large FSR is used and an array waveguide grating filter having a small FSR is used as the second filter, an optical frequency is roughly selected by the array waveguide grating filter having a large FSR, and the array waveguide having a small FSR is selected. It is possible to select finely with a lattice filter. The number of channels of these arrayed waveguide grating filters is M, N
By controlling the M + N optical gate, one channel can be selected from the number of M × N channels, and the number of electrodes to be controlled can be reduced even if the number of channels is large. It is possible to solve the problem (Japanese Patent Application No. 6-40657).

However, as described above, in a filter having a configuration in which an optical frequency filter using an arrayed waveguide grating and a gate is connected in two stages and the FSR is greatly different, some optical frequency It creates problems. This will be described below with reference to the drawings. FIG. 5 shows the transmission characteristics of a two-stage filter designed so that the FSR of the first optical frequency filter is four times that of the second optical frequency filter. FIGS. 5A, 5B, and 5C show the transmission characteristics of the first filter, the second filter, and the two-stage combined filter, respectively.

Here, if the frequency channel A1 is selected by the first filter and the channel B3 is selected by the second filter, the characteristic shown by A1 & B3 in FIG. 5C is obtained.
In this case, the resolution of the optical frequency depends on the resolution of the second filter having a small FSR and a narrow channel interval. Thus, while this filter seems to work well, it has the following problems: Consider the case where frequency channel A1 is selected by the first filter and channel B1 is selected by the second filter.
Is located at the end of the band of the channel A1, so that the change in the transmittance of the channel A1 is greatly changed.

As a result, as shown by A1 & B1 in FIG. 5 (c), the transmission characteristics of the entire filter are such that the transmittance is reduced and the frequency at which the transmittance is maximized is shifted from the transmission peak of the channel B1. It will be. That is, in a filter having a configuration in which filters having different FSRs are simply provided in two stages, there is a problem that the transmittance changes depending on the selected channel, and the transmission frequencies are not equal intervals.

[0017]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems in the prior art, and to reduce the number of control electrodes even when the number of selectable optical frequencies increases, and to select a channel to be selected. An object of the present invention is to provide an optical frequency filter in which a change in transmission characteristics is small even if the value is changed.

[0018]

According to the present invention, there is provided an optical frequency filter comprising a first and a second optical frequency selection filter arranged in series on the same substrate. The optical frequency selection filter section, a first arrayed waveguide grating filter having the function of splitting light incident on one input waveguide into at least two or more output waveguides for each different optical frequency, An optical gate switch for transmitting or blocking the demultiplexed light is connected between a second arrayed waveguide grating filter having an action of multiplexing the demultiplexed light into one waveguide. In the arrayed waveguide grating filters formed two each in the first and second optical frequency selection filter sections, demultiplexing or multiplexing between adjacent output waveguides or input waveguides. The waveguides are arranged such that the difference in the optical frequencies (so-called frequency channel spacing) is equal in all the arrayed waveguide grating filters, and the transmission optical frequency periodically generated in the first optical frequency selection filter unit is reduced. Adjacent to the two arrayed waveguide grating filters of the first optical frequency selection filter unit, so that the interval and the interval between transmitted light frequencies periodically generated in the second optical frequency selection filter unit are different by the difference of the optical frequency. The difference in length between matching array waveguides and the difference in length between adjacent array waveguides of two array waveguide grating filters in the second optical frequency selection filter are set to be different values. It is characterized by having been done.

[Operation] In the present invention, an optical frequency filter section composed of a pair of arrayed waveguide gratings and an optical gate switch array is connected in series, and a preceding optical frequency filter and a subsequent optical frequency filter are arranged. Are characterized in that the channel frequency intervals are equal to each other, and the FSR is different between the two by the channel interval.
In the optical frequency filter according to the present invention, by selecting a combination of the optical gate switch that performs the transmission operation of the optical frequency selection filter in the preceding stage and the optical gate switch that performs the transmission operation of the optical frequency filter in the subsequent stage, the light that can be selected with a small number of electrodes The number of frequencies can be significantly increased.

The principle will be described below. 5 channels, FSR as first optical frequency selection filter
Uses an arrayed waveguide grating filter five times the channel spacing, and has four channels as a second optical frequency selection filter.
FIG. 2A shows the transmission characteristics of each optical frequency selection filter when the FSR uses an arrayed waveguide grating filter whose channel interval is four times the channel interval and the channel intervals of both are equal.
(B) shows each. Numbers A1 to A shown in FIG.
Reference numerals 5, B1 to B4 denote optical gate switches that perform a transmission operation.

That is, the transmission distribution when the optical gate switches A1 and B1 are operated in the transmission operation and the other optical gate switches are in the cutoff operation is as shown by the solid line. In the first optical frequency selection filter unit, a transmission frequency band is periodically generated at every five channel intervals by performing the transmission operation of the optical gate switch A1. On the other hand, in the second optical frequency selection filter section, the optical gate switch B
By performing the transmission operation of 1, a transmission frequency band is periodically generated at every fourth channel interval. When light passes through the two-stage optical frequency selection filter unit, the transmission characteristics of the two are multiplied by the transmission characteristics of the entire optical frequency filter shown in FIG.

Therefore, the transmission characteristics of this filter are as shown in FIG.
As shown in (c), a transmission frequency band occurs every 20 channels (= 4 × 5). This optical frequency filter can be used as a filter that can select an arbitrary frequency from 20 channels. By changing the combination of the operation optical gates of the front and rear optical frequency selection filters, the selection of an arbitrary channel from the other 19 channels can be easily understood from these figures.

In the configuration of the conventional one-stage optical frequency filter, a filter capable of selecting an arbitrary frequency from among 20 channels requires control of 20 optical gate switches. = 5 + 4), it can be seen that the number of electrodes can be significantly reduced by controlling the optical gate switch. It is clear that the greater the number of required frequency channels, the greater the effect of the present invention in reducing the number of electrodes.

Further, since filters having the same channel interval between the two stages of filters, that is, filters having the same resolution, are used, the change in transmittance when the selected channel is changed is small.
The transmission frequency does not shift. Therefore, it is clear that the filter of the present invention has a uniform transmission characteristic that cannot be obtained by simply combining filters having different FSRs in two stages. Since the resolution of the filter is obtained by multiplying the characteristics of the two-stage filter, it is a feature of the filter according to the present invention that a filter characteristic with higher resolution can be obtained.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, an embodiment of the present invention will be described with reference to FIG. FIG. 1 schematically shows, from above, an optical frequency filter in which a first optical frequency selecting filter unit 1 and a second optical frequency selecting filter unit 2 are arranged in two stages in series on the same InP semiconductor substrate. It is. The optical frequency selection filter unit 1 has eight channels, and the optical frequency selection filter unit 2 has seven channels.

A large number of optical waveguides are formed on the InP semiconductor substrate, and the arrayed waveguide grating filter portions 3, 4, 6
The core layers of the waveguides of FIGS.
It is formed of 0 nm InGaAsP. The composition of the waveguide layers of the semiconductor amplifier units 5 and 8 used as optical gate switches has a band gap wavelength of 1550 to 1580 nm.
Of InGaAsP. Wavelength 1550 nm (optical frequency:
198 THz), or absorb or
Has an amplifying effect.

The voltage is controlled between the electrode 9 formed below the semiconductor amplifiers 5 and 8 and the electrode formed on the back surface of the substrate, and the current flowing through the waveguide layer of the semiconductor amplifier is turned on / off.
The light transmission / blocking is controlled by the FF operation. In the array waveguide grating filters 3 and 4 in the first optical frequency selection filter unit 1, the array waveguide, the slab waveguide, and the input / output waveguide are set so that the frequency channel interval is 100 GHz, the FSR is 800 GHz, and the number of channels is 8. Be placed. In the arrayed waveguide grating filters 6 and 7 in the second optical frequency selection filter unit 2, the frequency channel interval is 100 GHz, the FSR is 700 GHz, and the number of channels is 7
The array waveguide, the slab waveguide, and the input / output waveguide are arranged such that

Therefore, by using this filter, it is possible to extract any one light from the light having 56 optical frequency components arranged at regular intervals at a frequency interval of 100 GHz. Table 1 shows a correspondence table between the extracted light frequency and the operation gate that transmits light by passing a current. In this embodiment, the first optical frequency selection filter unit 1
Operation gates 1 to 8 and second optical frequency selection filter unit 2
191.0 T by the combination of the operation gates 1 to 7
Light of an arbitrary channel can be extracted from 56 frequency channels within a frequency range from Hz to 196.5 THz. The number of electrodes that need to be controlled is 15
It has become.

[0029]

[Table 1]

[0030]

As described above, according to the present invention, the number of optical frequencies (wavelengths) that can be selected by controlling a small number of electrodes is remarkably increased. It is possible to realize an optical frequency filter with less deviation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an embodiment of the present invention.

2A is a transmission spectrum of the optical frequency selection filter units A1 to A5, FIG. 2B is a transmission spectrum of the optical frequency selection filter units B1 to B4, and FIG. c) is a graph showing the transmission spectrum of the entire optical frequency filter.

FIG. 3 is a configuration diagram of a conventional example.

FIG. 4 is a configuration diagram showing a two-stage Mach-Zehnder filter.

5 (a) shows transmission spectra of optical frequency selection filter units A1 and A2, and FIG. 5 (b) shows optical frequency selection filter units B1 to B2 when optical frequency selection filters having different FSRs are provided in two stages. Transmission spectrum of B4, FIG.
(C) is a graph showing the transmission spectrum of the entire optical frequency filter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st optical frequency selection filter part 2 2nd optical frequency selection filter part 3,6 Array waveguide grating splitter 4,7 Array waveguide grating multiplexer 5,8 Semiconductor amplifier (optical gate switch) 9 Electrode for controlling optical gate 10 Arrayed waveguide grating demultiplexer 11 Arrayed waveguide grating multiplexer 12 Semiconductor amplifier array 13, 18 Input optical waveguide 16, 21 Output optical waveguide 14, 15, 19, 20 Slab waveguide 17, Reference Signs List 22 array waveguide 23 optical gate control electrode 24 InP substrate 25, 33 input light 26, 34 output light 30 first Mach-Zehnder filter 31 second Mach-Zehnder filter 32 control electrode

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuo Shibata 3-19-2 Nishi Shinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Yuzo Yoshikuni 3- 192-1 Nishi-Shinjuku, Shinjuku-ku, Tokyo No. Nippon Telegraph and Telephone Corporation F-term (reference) 2H079 AA02 AA13 BA01 CA05 DA16 EA07 EB04 HA07 HA14 5F073 BA01 CA12 CB02 GA38

Claims (1)

[Claims] 1. An optical frequency filter in which first and second optical frequency selection filter units are arranged in series on the same substrate, wherein the optical frequency selection filter unit includes a light incident on one input waveguide. A first arrayed waveguide grating filter having the function of splitting the light into at least two or more output waveguides for each different optical frequency, and a second array filter having an action of combining the split light into one waveguide. An optical gate switch for transmitting or blocking the demultiplexed light is connected between the two arrayed waveguide grating filters, two in each of the first and second optical frequency selection filter units. In the formed arrayed waveguide grating filter, the difference in optical frequency demultiplexed or multiplexed between adjacent output waveguides or input waveguides becomes equal in all the arrayed waveguide grating filters. The waveguide is arranged as described above, and the interval between the transmitted light frequencies periodically generated in the first optical frequency selection filter unit and the interval between the transmitted light frequencies periodically generated in the second optical frequency selection filter unit are the aforementioned. The difference in length between the adjacent array waveguides of the two array waveguide grating filters of the first optical frequency selection filter unit and the difference in the length of the two An optical frequency filter, wherein a difference in length between adjacent array waveguides of an arrayed waveguide grating filter is set to a different value.