JP2648921B2 - Frequency locking method and apparatus and optical broadband communication network - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and an apparatus for fixing the frequency of a plurality of coherent radiation beams to a fixed value.

(Background technology)

2. Description of the Related Art An optical communication device that multiplexes and transmits optical frequencies in close proximity has been developed. These devices can transmit a large number of optical information channels through a single optical waveguide such as an optical fiber, and can transmit a large number of channels in a narrow frequency band of the fiber. Frequency multiplexing is performed using a plurality of laser transmitters operating at close frequencies and sent to a demultiplexer via a single fiber. Individual channels can be detected using a tunable heterodyne receiver or possibly a simple optical filter.

Accurate control of the laser frequency is important to maximize the number of available channels.

[Disclosure of the Invention]

The present invention is to solve the above problems, a first invention of the present invention, in a frequency lock method for maintaining the frequency of the plurality of first coherent radiation beam at a predetermined value,
Generating a second reference coherent radiation beam comprising a plurality of reference frequencies, for each said first coherent radiation beam, monitoring the relationship between that frequency and a corresponding one of said reference frequencies, the monitored frequency The frequency of the first coherent radiation beam is modified so that the relationship is substantially constant.

A second invention of the present invention is a frequency lock device that maintains the frequencies of a plurality of first coherent radiation beams at a predetermined value, and a reference beam generator that generates a second reference coherent radiation beam including a plurality of reference frequencies. For each of the first coherent radiation beams, monitoring means for monitoring the relationship between the frequency and one of the corresponding reference frequencies to generate a control signal; and in response to the control signal, Correction means for correcting the frequency of the coherent radiation beam such that the frequency relationship monitored by the means is substantially constant.

The present invention solves the above described problem by locking each first beam to its corresponding reference frequency, where the reference frequency is provided by a reference coherent radiation beam.

There are many ways to perform the monitoring steps. For example, a reference coherent radiation beam may be split into a plurality of auxiliary beams, and the auxiliary beam may be filtered to remove other frequencies, leaving only one reference frequency, such that each auxiliary beam has a single frequency or a small frequency band. Remove.

However, the monitoring step includes distributing the reference beam into a plurality of auxiliary reference beams, generating an intermediate beam including a plurality of intermediate frequencies by interfering one auxiliary reference beam with each first beam, and generating an intermediate beam including a plurality of intermediate frequencies. On the other hand, it is desirable to monitor the one intermediate frequency, wherein the modifying step modifies the temporary beam frequency such that the intermediate frequency is substantially constant.

If the radiation is light, the reference beam can be distributed using a conventional fiber optic coupler, a planar waveguide coupler, or a prism beam splitter. Therefore, the monitoring means, a beam splitter for distributing the reference beam into a plurality of auxiliary reference beams, and coupling means for causing each primary beam to interfere with one auxiliary beam to generate an intermediate beam including a plurality of intermediate frequencies, Detection means for detecting one intermediate frequency for each intermediate beam and generating a control signal associated with the detected intermediate frequency.

The frequency of the first beam may be corrected by controlling a laser or other beam source, or the beam may be passed through a frequency modulator controlled by a control signal generated by the detection means.

The second reference coherent radiation beam provides a "comb" -type distributed reference frequency, which is preferably distributed at equal frequency intervals about the main reference frequency. Conveniently, such a reference coherent radiation beam can be generated by modulating one or more main reference coherent radiation beams with a radio frequency signal, which can generate multiple reference frequencies.

A Mach-Zehnder element or an optical phase modulator is suitable as a reference beam generator that outputs a main reference coherent radiation beam. In either case, modulation can be performed by driving the element at radio frequency.

If each first beam interferes with a reference coherent radiation beam containing a reference frequency, multiple intermediate frequencies are generated. It is necessary to lock each individual first coherent radiation beam to a different reference frequency, for which it is convenient to identify the reference frequency from which each intermediate frequency is generated. If the modulated reference coherent radiation beam is further modulated at a relatively lower frequency, the frequency of the intermediate beam will be differently shifted depending on the included reference frequency and will include the desired shift in the monitoring step By monitoring the intermediate frequencies, each reference frequency can be easily identified.

This additional modulation can be achieved by modulating the radio frequency before modulating the main reference frequency. Also, at a remote location, the main reference coherent radiation beam may be modulated by a radio frequency and another modulation signal.

The radio frequency modulated signal is periodic, eg, sinusoidal, or may include a pulse train. Further, two or more sine wave signals may be combined to form a modulation signal.

The reference beam generator is a suitable laser, for example 1.5 μm
HeNe laser for band, Nd-YAG for 1.3 μm band
It is suitable to include a laser or a semiconductor laser locked to a suitable atomic standard, such as an absorption line.

Although the invention is particularly suited for implementation in the optical domain, it can also be used in certain non-optical wavelength bands. In this specification,
Using the term "light" as part of the electromagnetic spectrum,
It includes not only the visible region but also infrared and ultraviolet regions on both sides of the visible region that can be transmitted by a dielectric optical waveguide such as an optical fiber.

The term "coherent" is also used in this specification, since the term "coherent" is used in the field of communication devices in a sense that the spectral line width of the beam is small compared to the bandwidth of modulation by information.

Also, the term “frequency” may be replaced with the term “wavelength”, but there is an inverse relationship between them.

An example of an optical communication device provided with a frequency lock device for performing the method of the present invention will be described with reference to the drawings.

[Brief description of drawings]

 FIG. 1 is a block diagram of a communication device.

FIG. 2 is a block diagram showing a part of the frequency lock device.

3 to 5 are diagrams showing signal waveforms at respective points in the apparatus shown in FIG.

FIG. 6 is a diagram showing a frequency response of the frequency discriminator shown in FIG.

 FIG. 7 is a diagram showing application of reference frequency identification modulation.

[Best mode for carrying out the invention]

An optical communication network incorporating an example of a frequency lock device according to the present invention is schematically illustrated in FIG. The communication device includes a plurality of n transmitters 1, which are connected by an optical fiber 2 to an optical coupler, that is, a multiplexer 3. The optical signal from the optical fiber 2 is frequency-division multiplexed into one optical fiber 4 by a multiplexer 3 and connected to a remote optical splitter, that is, a demultiplexer 5.
The demultiplexer 5 distributes the incoming signal to a plurality of optical fibers, which are connected to respective corresponding receivers 7. Each receiver 7 has a local oscillator,
By a general method, an incoming signal can be detected and converted into an electrical demodulated information signal.

Each transmitter 1 also includes a laser or local oscillator, the output signal of which is modulated by the information signal indicated by arrow 8.

In this type of communication network, it is necessary to accurately control the frequency of the local laser in the transmitter 1. For this purpose, a reference laser 9 is provided. The details are shown in FIG. (Alternatively, instead of the reference laser 9, a reference laser device that uses a plurality of reference lasers and multiplexes their outputs with each other may be used. The wavelength of the reference laser is 100 GHz.
The interval. ) Reference laser is HeNe operating at 1.523μm.
Laser 10 is a LiNbO ₃ waveguide Mach-Zehnder interferometer via a mechanical deflection control element 13 by an optical fiber 11
Combined with 12. The interferometer 12 includes two phase modulators arranged in parallel, and these phase modulators are connected in a Y-shape at an input end and an output end. Radio frequency source 4 is provided for supplying a radio frequency signal of frequency f ₀ to the interferometer 12. If the input power (f _R ) is evenly distributed at the input Y-junction, this element is modulated at frequency f ₀ to a phase modulation depth φ ₁ and the two signals are output to the output Y-shape. Recombination at the mold junction produces a DC phase difference φ ₀ between the two signals. At this time, generally, the generated light signal of a frequency f _R ± nf ₀ is the _{intensity, I (f R ± nf 0} ) αJ n 2 (φ 0) - a [1 + (1) ⁿ cosφ _0] . Here, J _n is a Bessel coefficient of the first kind of order n. By precisely adjusting the input polarization (polarization control element 13), radio frequency power and DC bias, a comb-shaped frequency distribution including the fundamental frequency and the first, second, third,...

As an example, FIG. 3 shows a spectrum obtained when the modulation signal is 420 MHz at +28 dBm and the DC bias is about 7 V. This spectrum is measured directly at the output end of the Mach-Zehnder interferometer 12 by scanning with an etalon-type optical spectrum analyzer.

Therefore, the interferometer 12 operates as a comb spectrum generator.

The frequency distributed in the form of a comb from the interferometer 12 is supplied to the optical power splitter 15, whereby the input beam is distributed and supplied to the transmitter 1 shown in FIG. For simplicity, FIG. 2 shows only one of the transmitters 1 in detail.

Each transmitter 1 includes an optical coupler 6 to which the incoming signal from the reference laser device 9 is supplied. The local laser of the transmitter 1 is an external cavity laser 17, for example, composed of an InGaAsP diode laser, provided with an antireflection coating on the end face and coupled to an external cavity 10 cm long. This external resonator includes a diffraction grating of 1200 lines / mm and a silica plate.
, The resonator length can be finely adjusted.

The output of the laser 17 is supplied to the optical splitter 20 via the polarization control device 19. A portion of the laser beam is provided by a splitter 20 along an optical fiber 21 to a general information modulating means (not shown), which in turn supplies the multiplexer 3 for transmission. . The remaining portion of the laser beam is provided to a second input port of an optical coupler 16 where it is mixed with the reference beam from splitter 15. The mixed signal is provided to a PINFET photodiode receiver 22. The polarization control element 19 is adjusted so that the polarization is matched at the receiver 22.

The coupling by the coupler 16 mixes the signal from the laser 17 into each of the comb-distributed frequencies, producing a large number of intermediate frequencies if the receiver bandwidth is wide enough. These intermediate frequencies are amplified by the amplifier 23 and supplied to the intermediate frequency filter 24. A part of the output from the filter 24 is supplied to a path delay frequency discriminator 25, and an output signal of the frequency discriminator 25 is supplied to a driving circuit via a splitter 30.
Control 18 Therefore, the filter 24 and the frequency discriminator 25 together with the optical path from the drive circuit 18 and the laser 17 form a frequency lock loop.

The frequency at which the laser 17 is locked is defined by the settings of the filter 24.

An experiment was conducted on the configuration shown in FIG. 2, and the position of the diffraction grating associated with the laser 17 was adjusted, and the wavelength of the external cavity laser was brought closer to the frequency of each tooth of the comb-shaped distribution.・ The loop was closed and the frequency was locked. In this experiment, five comb teeth arranged at intervals of 420 MHz were used, and the intermediate frequency was initially set to 100 MHz. FIG. 4 shows the fundamental wave measured by scanning the etalon and the first and second harmonics on both sides thereof. The device used for the experiment prevented locking to the third harmonic in advance. The frequency lock was stable and could be maintained for several hours without interference. The frequency jitter introduced by the control loop is 5 MHz for the fundamental and 6 MHz for the first harmonic
z, 7 MHz at the second harmonic. This value is greatly affected by the intensity of the intermediate frequency. Basically, either side of each tooth can generate the same intermediate frequency to lock the external cavity laser, but this was avoided.
The reason is that, as shown in FIG. 6, the frequency discriminator has a periodic characteristic in which the lock points are arranged asymmetrically around 0 Hz. In this characteristic, f ₁ is measured as + 102MHz, f ₂ is measured with -327MHz, f ₃ was measured as + 535 MHz. Since f ₂ and f ₃ are disengaged from the band of the intermediate frequency filter, only frequency lock when the intermediate frequency is f ₁ occurs. If the intermediate frequency filter is replaced with a filter centered on 320 MHz, the frequency can be locked to the fundamental wave offset by 327 MHz as shown in FIG.
By changing the center frequency of the filter 24 using the characteristics of the frequency discriminator 25, the frequency can be slightly offset. This allows the frequency control loop to be locked to a different intermediate frequency. When the frequency discriminator 25 is designed such that the interval between the series of intermediate frequencies is smaller than the frequency interval of the light comb distribution, this technique is used to finely separate the teeth between the teeth of the light distribution. Stepwise tuning can be performed. By adjusting the point on the slope where the loop is locked, a fine tuning that is continuous in steps,
If possible, it can be tuned to the center of the fine step.

In the control loop, it is necessary to know to which tooth of the comb distribution the frequency is locked. This is obtained by defining the position of the tooth relative to the fundamental. Some identification can also be performed on each tooth. this is,
For example, it is obtained by modulating the offset signal f ₀ with a low frequency signal (f _M ). With such modulation,
The deviation (FIG. 7) that appears at each tooth (within each sideband) of the comb distribution is different, and the higher the order, the greater the deviation. To detect this in the control loop, a part of the output of the filter 24 is led to the identifier checking circuit 31 via the splitter 20. If a simple detector is used, the deviation must be greater than the jitter generated by the control loop. Also, it must be smaller than the frequency of the modulation signal in the channel. When the deviation cannot be greater than the loop jitter, a correlation receiver is needed to detect low frequency modulation.

Coarse tuning can be performed by the signal output from the identifier checking circuit 31, and fine tuning can be performed by the output from the frequency discriminator 25.

Although the communication device shown in FIG. 1 has a very simple structure, the invention can be used with more complicated communication devices. For example, the output signal from the reference laser device 9 can be multiplexed on another fiber (not shown) or multiplexed with the transmission signal on the fiber 4 and additionally supplied to the receiver 7. Further, a transmitter (not shown) may be provided in the receiver 7 to form a two-path communication device. In this case, these transmitters are locked to the reference frequency transmitted from the reference laser device 9.

For a given order, both the high and low frequency sidebands of the comb distribution may produce the same frequency deviation and become ambiguous. Further, the loop containing the controlled laser 17 may be locked above or below the selected order. To solve this, circuits are provided around the control loop to check the position and deviation of adjacent orders.

FIG. 7 also shows the channel frequency 26
Shows an example in which is offset by the frequency F _IF . Channel frequency is given by f _R -2f ₀ + F _IF.

Continuation of the front page (72) Inventor Funkin David John 22nd Aid Suffolk Woodbridge Devenrod, UK I-121 (56) References JP-A-61-87383 (JP, A)

Claims (10)

(57) [Claims] 1. A frequency locking method for maintaining a frequency of each of a plurality of first coherent radiation beams at a predetermined value, the method comprising: generating one second reference coherent radiation beam including a plurality of reference frequencies; Monitoring, for each of the first coherent radiation beams, a relationship between the frequency and one of the plurality of reference frequencies corresponding to the first coherent radiation beam; and Modifying the frequency of each of the plurality of first coherent radiation beams to be substantially constant. 2. The method of claim 1, further comprising: distributing the second reference coherent radiation beam to a plurality of auxiliary reference beams; and causing each of the plurality of first coherent radiation beams to interfere with one of the plurality of auxiliary reference beams. Generating an intermediate beam including a plurality of intermediate frequencies; and, for each of the intermediate beams obtained for the plurality of first coherent radiation beams, monitoring one of the intermediate frequencies included in the intermediate beam. 2. The method of claim 1, wherein the modifying step comprises modifying a frequency of each of the plurality of first coherent radiation beams such that each monitored intermediate frequency is substantially constant. Frequency lock method. 3. The method of claim 1, wherein generating the second reference coherent radiation beam comprises modulating one or more main reference coherent radiation beams at a radio frequency to generate a plurality of reference frequencies. 3. The frequency lock method according to item 2. 4. A method for generating a second reference coherent radiation beam comprising: modulating one or more main reference coherent radiation beams at a radio frequency to generate a plurality of reference frequencies; Further modulating at a relatively low frequency to produce a reference frequency dependent deviation in the frequency of the intermediate beam, wherein the monitoring comprises monitoring the intermediate frequency with the desired deviation. 3. The frequency lock method according to claim 2. 5. The frequency locking method according to claim 1, wherein the first coherent radiation beam and the second reference coherent radiation beam are light beams. 6. A frequency lock device for maintaining respective frequencies of a plurality of first coherent radiation beams at a predetermined value, comprising: a reference beam generator for generating one second reference coherent radiation beam including a plurality of reference frequencies; Monitoring means for monitoring the relationship between the frequency of each of the plurality of first coherent radiation beams and one of the plurality of reference frequencies corresponding to the first coherent radiation beam to generate a control signal; And correcting means for correcting the respective frequencies of the plurality of first coherent radiation beams so that the respective frequency relations monitored by the monitoring means are substantially constant in response to the control signal. A frequency lock device. 7. A monitoring means, comprising: a beam splitter for distributing a second reference coherent radiation beam to a plurality of auxiliary reference beams; and causing each of the plurality of first coherent radiation beams to interfere with one of the plurality of auxiliary reference beams. Coupling means for generating an intermediate beam including a plurality of intermediate frequencies, for each of the intermediate beams obtained for the plurality of first coherent radiation beams, to detect one intermediate frequency included in the intermediate beam, The frequency lock device according to claim 6, further comprising: a detection unit configured to generate a control signal related to the intermediate frequency. 8. A reference beam generator, comprising: one or more main reference beam generators; a radio frequency signal source; and a radio frequency signal source supplied from the radio frequency signal source. The frequency lock device according to claim 6, further comprising: a modulator that modulates with a frequency. 9. A first beam source for generating a first coherent radiation beam, the first beam source responsive to a control signal generated by the monitoring means for controlling a frequency of each first coherent radiation beam. The frequency lock device according to any one of claims 6 to 8, wherein the frequency lock device is configured to correct. 10. A plurality of transmitters each connected to a coherent optical signal source, multiplexing means for frequency-division multiplexing or wavelength division multiplexing an output signal of the transmitter, and a frequency of a signal generated by the signal source is determined. Means for maintaining a value of the reference beam; wherein the means for maintaining comprises: a reference beam generator for generating one second reference coherent radiation beam including a plurality of reference frequencies; and for each of the plurality of first coherent radiation beams, Monitoring means for monitoring a relationship between the frequency and one of the plurality of reference frequencies corresponding to the first coherent radiation beam to generate a control signal; and in response to the control signal, The respective frequencies of the plurality of first coherent radiation beams such that the respective frequency relationships monitored by the means are each substantially constant. Modifying means for modifying; and an optical broadband communication network.