JPH10150235A - Reference light frequency generating circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reference light frequency generating circuit which is capable of stably and continuously feeding a light pulse train which serves as a light frequency reference from a light ring circuit. SOLUTION: A reference light frequency generating circuit is equipped with a light pulse modulator 1 which modulates continuous light inputted from outside into a repetitive pulse train, a light ring circuit 2a, and an optical branching filter 3. An optical amplifier 4, a light frequency shifter 5, a light switch 6, a light delay line 7, and a depolarizer 8 are arranged on the optical path of the light ring circuit 2a, a synchronism control system 9 which controls the delay synchronism of modulation signals and open/close signals which are sent to the light pulse modulator 1 and the light switch 6 respectively is provided, and an optical branching filter 10 which is used for taking out light that propagates through the light ring circuit 2a is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reference light frequency generating circuit for stably generating and supplying a large number of reference light frequencies.

[0002]

2. Description of the Related Art As a technique for generating a plurality of reference optical frequencies, there is known a method of using an optical ring circuit including an optical amplifier, an optical frequency shifter, an optical delay line, and an optical switch therein (see References). Reference: K. Shimizu et al., “Frequency t
ranslation of lightwaves bypropagation around an o
ptical ring circuit containing a frequency shifte
r: Part I. Experiment, "Appl. Opt., vol.32, no.15,
p. 6718, 1993.). FIG. 3 shows an example. The light wave subjected to absolute frequency stabilization using an absorption line or the like is pulsed by the light pulse modulator 1 and then input to the light ring circuit 2 via the light directional coupler 11. At this time, the time width of the optical pulse and the round trip time of the optical ring circuit 2 are matched using the optical delay line 7 or the like, and the gain of the optical amplifier 4 is adjusted so as to compensate for the loss of the entire optical ring circuit 2. By doing so, the inputted optical pulse can be circulated many times. The light pulses, for receiving a constant frequency shift f _s for each one circuit by passing through the optical frequency shifter 5,
After many rounds, the frequency of the light pulse undergoes a large frequency shift proportional to the number of rounds. The stability of the frequency shift f _s by the optical frequency shifter is very high,
Since the absolute frequency of the optical pulse is frequency-converted and frequency-swept without impairing the original stability, these orbiting optical pulses can be newly used as an optical frequency reference. Further, by performing the delay synchronization control of the pulse modulation signal to the optical pulse modulator 1 and the opening and closing of the optical switch 6 in the optical ring circuit 2, the frequency sweep can be repeated as shown in FIG. That is, while the optical switch 6 is in the open state, the circulation of the optical pulse is repeated and the frequency conversion is performed. However, since noise light accumulates during multiple rounds, the optical switch 6 is once closed to interrupt the round, and an optical pulse is newly cut out by the optical pulse modulator 1 and rounded. By repeating these series of operations, a reference light frequency pulse having a specific absolute frequency can be periodically generated.

[0003]

However, the spontaneous emission noise light generated inside the optical amplifier 4 increases and accumulates along with the circulation, and the light pulse due to the combined effect of the loss of the optical ring circuit 2 and the polarization dependence. Instability of the lap. Specifically, assuming that one polarization eigenmode vector of the optical ring circuit 2 is _X, the loss coefficient L _X of the optical ring circuit 2 for the polarization eigenmode _X is equal to the optical ring circuit for the polarization eigenmode Y orthogonal thereto. for loss coefficient L is smaller than _Y, that is, the loss on the polarization eigenmodes X is greater than the loss on the polarization eigenmodes Y has a problem that circulation of the light pulse becomes unstable. Hereinafter, this cause will be briefly described.

[0004] Assuming that the optical pulse is in the polarization eigenmode X, and the number of rotations of the optical pulse is represented by n and the gain of the optical amplifier 4 is represented by G, the intensity P _s ^X (n) of the optical pulse after n rotations is obtained. Is
It is given by the following equation.

(Equation 1)

Here, the value of the net gain L _X G (n) is 1
If it is only slightly smaller, multiple rounds of the light pulse are possible, but if it is smaller than 1 by several percent or more, the light pulse decreases exponentially and disappears with the round. On the other hand, since the optical amplifier 4 operates in the gain saturation region, the optical pulse P _S ^X (n) and the spontaneous emission noises P _N ^X (n) and P _N ^Y (n) corresponding to the X and Y polarization eigenmodes are used. total strength of, be kept on average at a constant value P _C in determined by the pump intensity of the optical amplifier 4 regardless of the number of cycles n is experimentally known (see above reference). That is, the following equation holds.

(Equation 2)

From equation (2), the gain of the optical amplifier 4 is

(Equation 3)

The gain L _X G of the optical ring circuit 2 with respect to the optical pulse is given by

(Equation 4)

[0008] Here, if L _X <L _Y , that is, if the loss for the optical pulse in the polarization eigenmode X is greater than the loss for the spontaneous emission noise light having the polarization eigenmode Y, the net of the optical ring circuit 2 for the optical pulse the gain L _X G will be below 1. Therefore, depending on the magnitude of the second term of the denominator in equation (4), the intensity of the light pulse decreases exponentially with the circulation as shown in equation (1). Therefore, when the polarization eigenmode vector changes due to the change in birefringence in the optical ring circuit and the difference between the values of the loss coefficients corresponding to the two orthogonal polarization eigenmodes increases, the rotation of the optical pulse Could be unstable.

The present invention has been made under such a background, and an object of the present invention is to provide a reference light frequency generating circuit capable of stably supplying an optical pulse train serving as an optical frequency reference from an optical ring circuit. And

[0010]

According to the present invention, there is provided an optical pulse modulating means for modulating continuous light input from the outside into a repetitive pulse train, an optical ring circuit, and an optical pulse circuit. An optical input means for inputting the output light from the modulation means to the optical ring circuit, an optical amplifier, an optical frequency shift means, an optical switch, an optical delay line, and a depolarizer on an optical path of the optical ring circuit. A synchronization control system for delay and synchronous control of a modulation signal for the optical pulse modulation means and an opening / closing signal for the optical switch, and an optical output means for extracting propagation light propagating through the optical ring circuit to the outside. It is characterized by having.

As the depolarizer, two polarization maintaining optical fibers each having a length of 1: 2 and having a birefringent optical axis connected to each other by 45 degrees can be used.

Further, the period N of the fluctuation of the polarization eigenmode vector of the light pulse may be an integer.

According to the present invention, a depolarizer is inserted into an optical ring circuit so that not only the optical frequency of an optical pulse but also the polarization eigenmode vector changes along with the circuit. The loop instability caused by the combined effect of the polarization state of the pulse and the polarization direction dependence of the loss of the optical ring circuit was reduced. The operation will be described below.

As the depolarizer, the length L and its 2
It is assumed that two birefringent optical waveguides having a double length 2L are connected such that the birefringent optical axes are at 45 degrees to each other. The refractive indices of the birefringent waveguide for the two linear polarization modes are n _f and _ns , and the input ends correspond to the linear polarization modes X and Y, respectively. Linear polarization X, respectively E _X the electric field components of light corresponding to Y, when displayed decomposed into E _Y, changes in the polarization state by passing through the depolarizer is expressed by the following equation.

[0015]

(Equation 5)

Here, ν represents the optical frequency, and C represents the speed of light in a vacuum. By passing through the depolarizer, there is a new maximum between the X and Y components of the electric field.

(Equation 6)

Minimum

(Equation 7)

The following phase difference occurs. The two electric field components E _X orthogonal phase difference E _Y is results in a change in the polarization state of output light, the change in polarization state input to the magnitude of the phase difference is also dependent on the optical frequency ν It depends on the frequency of the light pulse. At this time, the polarization conversion matrix in the equation (5) also changes as the optical frequency changes with the rotation, so that the polarization conversion matrix of the optical ring circuit no longer has constant eigenvalues / eigenvectors. The polarization eigenmode vector changes for each rotation. Therefore, a certain polarization eigenmode and a polarization eigenmode orthogonal to each other are introduced as eigenpropagation modes of the optical ring circuit, and an arbitrary electric field vector is expressed as a linear sum of those components, and as shown in Expression (1), It becomes impossible to calculate the change of the electric field due to the circulation for each polarization eigenmode component. Conversely, this implies that an exponential decrease in the light pulse intensity with respect to the number of rounds n, which occurs when L _X G (n) -11 <0 in Equation (1), cannot occur in principle. Means

The polarization eigenmode vectors at the number of rotations m are X _m and Y _m , and the corresponding loss coefficients are L _Xm and L _Xm , respectively.
When _{Ym (L Xm <L Ym)} , component mode X _m will suffer more loss than the components of the mode Y _m.
In the next m + 1 round, the polarization eigenmode vector changes to X _{m + 1} and Y _{m + 1} due to the frequency shift, and the corresponding loss coefficients are also L _{Xm + 1} and L _{Ym + 1} (L _Xm <L _Ym )
Changes to Here, part of the light intensity in the _Xm mode with a large loss in the m-th round is distributed to the L _{Ym + 1} mode with a small loss in the (m + 1) -th round. In addition, a part of the light intensity in the Y _m mode with the small loss in the m-th round is distributed to the L _{Xm + 1} mode with the large loss in the m + 1 round. In this way, the influence of the net gain difference caused by the polarization dependence of the loss due to the polarization eigenmode vector changing with the rotation is averaged during one rotation of the polarization eigenmode vector.

The period N of the change of the polarization eigenmode vector of the light pulse caused by the depolarizer (the rotation of the polarization eigenmode vector makes one rotation in N rounds) can be estimated as follows.

In equation (7), the change amount Δν of the optical frequency such that the phase difference on the left side is 2π

(Equation 8)

[0022] Assuming that a frequency shift per time is Δf, Δν = NΔf.

(Equation 9)

## EQU2 ## Light velocity C, birefringence difference (n _f
-N _s), the length ^{L, 3X10 8 m / s,} 10 -5, 10 3
By substituting m = 1 km for each, the value of Δν is:
Obtain 30 GHz. Here, a typical value in the polarization maintaining optical fiber is used as the birefringence difference. If the frequency shift Δf per time is 100 MHz, the value of N is estimated to be approximately 30 times.

Since the two polarization eigenmode vectors of the optical ring circuit with respect to the optical pulse make almost one rotation during the 20 to 30 rotations, the net gain caused by the polarization direction dependence of the loss of the optical ring circuit is obtained. Is averaged over time while the optical pulse makes 20 to 30 rotations. Therefore, it is possible to remarkably reduce the instability of the circulation of the optical pulse caused by the combined effect of the polarization state of the optical pulse and the polarization dependence of the loss of the optical ring circuit. Further, by reducing the number of rounds required for one rotation of the polarization eigenmode vector, the stability of the rounding of the optical pulse is expected to be improved.

[0025]

FIG. 1 is a diagram for explaining an embodiment of the present invention. Reference numeral 1 denotes an optical pulse modulator as an optical pulse modulator for repeatedly modulating continuous light input from the outside into a pulse train, 2a denotes an optical ring circuit, and 3 denotes an optical output from the optical pulse modulator 1. An optical splitter as an optical input means for inputting to the ring circuit 2a. In addition, as the light input means, an optical directional coupler or the like can be used in addition to the bulk optical splitter. 4 is an optical amplifier, 5 is an optical frequency shifter as an optical frequency shift means, 6 is an optical switch, 7 is an optical delay line, 8 is a depolarizer, and these optical elements are inserted in series in an optical ring circuit. . Specifically, an optical fiber amplifier, a semiconductor laser amplifier, or the like is used as the optical amplifier 4, and an acousto-optic frequency shifter (shift amount of 100 MHz to several hundred MHz) is used as the optical frequency shifter 5.
z) and the like; an optical fiber as the optical delay line 7; two birefringent optical waveguides having a length L and a length 2L twice as long as the depolarizer 8;
A birefringent medium or a birefringent optical fiber which is connected in an appropriate manner is used.

Assuming that L = 300 m, 2L = 600 m, and a birefringence index difference of 3 × 10 ⁻⁵ as an example in the case of using a birefringent optical fiber, the case described in the section “Means for Solving the Problems” is the same as that described above. Similarly, the amount of frequency shift required for one rotation of the polarization eigenmode vector is about 30 GHz. Reference numeral 9 denotes a synchronization control system, which delay-synchronizes a modulation signal for the optical pulse modulator 1 and an open / close signal for the optical switch 6 to repeatedly control frequency sweeping along with the rotation of the optical pulse with respect to time. Reference numeral 10 denotes an optical splitter as an optical output unit for extracting propagation light propagating through the optical ring circuit 2a to the outside, from which an optical pulse train whose frequency is swept stepwise with respect to time is output stably. . However, the polarization state of the light pulse changes according to the number of rounds. In addition,
As the output means, an optical directional coupler can be used in addition to the bulk optical splitter.

Next, another embodiment of the reference light frequency generating circuit will be described with reference to FIG. FIG. 2 shows a configuration in which a depolarizer 8 similar to the above is newly inserted into the ring circuit 2 of the conventional reference optical frequency generation circuit shown in FIG. Comparing this embodiment with the embodiment described with reference to FIG. 1, FIG. 1 shows that the optical input means (optical splitter 3) and the optical output means (optical splitter 10) are provided on the optical ring circuit 2a. Are provided separately, whereas the optical ring circuit 2b of FIG.
The difference is that the light input means and the light output means are provided as one unit by using the light directional coupler 11. Another difference is that the components inserted in the optical ring circuit have different configuration orders. That is, the configuration order of each component need not be limited to that shown in FIG. 1 or FIG.

[0028]

As described above, according to the present invention, an externally pulsed absolute frequency stable signal is output to an optical ring circuit including an optical amplifier, an optical frequency shift means, an optical delay line, a depolarizer and an optical switch. An optical pulse train, which serves as an optical frequency reference,
It becomes possible to continue supplying more stably than before.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a reference frequency generation circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a reference frequency generation circuit according to another embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of an optical ring circuit for generating a conventional optical reference frequency pulse train.

FIG. 4 is an explanatory diagram for explaining the concept of repetitive optical frequency sweep.

[Explanation of symbols]

1: light pulse modulator (light pulse modulation means), 2a, 2
b: optical ring circuit, 3: optical splitter (optical input means), 4:
Optical amplifier, 5: Optical frequency shifter (optical frequency shift means), 6: Optical switch, 7: Optical delay line, 8: Depolarizer, 9: Synchronous control system, 10: Optical splitter (optical output means), 11: Optical directional coupler (optical input, optical output means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/00 (72) Inventor Hiroki Takei 3-2-19-1 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. An optical pulse modulator for repeatedly modulating continuous light input from the outside into a pulse train; an optical ring circuit; and an optical input for inputting output light from the optical pulse modulator to the optical ring circuit. Means, an optical amplifier, an optical frequency shift means, an optical switch, an optical delay line, and a depolarizer are arranged on an optical path of the optical ring circuit, and a modulation signal for the optical pulse modulation means and A reference optical frequency generation circuit, comprising: a synchronization control system for delay-locked control of an opening / closing signal for an optical switch; and an optical output unit for extracting propagation light propagating through the optical ring circuit to the outside. 2. The depolarizer according to claim 1, wherein two polarization maintaining optical fibers each having a length of 1: 2 are connected such that their birefringent optical axes are at 45 degrees to each other. Item 2. A reference optical frequency generation circuit according to Item 1. 3. The reference light frequency generating circuit according to claim 2, wherein the period N of the fluctuation of the polarization eigenmode vector of the light pulse is an integer.