JP5063900B2 - Raman gain measuring method and apparatus - Google Patents

Description

  The present invention relates to a method and apparatus for measuring Raman gain.

  When strong light (pump light) is input to an optical fiber made of a nonlinear optical material such as silica glass, optical amplification occurs in a wavelength band longer by 13 THz (about 100 nm) than the pump light due to stimulated Raman scattering. This light amplification effect can amplify signal light with low noise. This amplifier is called a Raman amplifier. In recent years, Raman amplifiers have attracted attention as a means for extending the transmission distance by short-to-medium distance optical fiber communication.

  The gain of the Raman amplifier is called Raman gain. In addition, when 1 W of pump light is input to the Raman amplification medium, the Raman gain (dB) received by the signal light propagating through the Raman amplification medium is referred to as Raman gain efficiency (dB / W). The Raman gain efficiency depends on the mode field diameter of the optical fiber serving as the Raman amplification medium, the additive and amount thereof, water absorption, and the like.

In order to design or actually use a Raman amplifier, it is necessary to accurately measure Raman gain or Raman gain efficiency, or Raman gain coefficient. The Raman gain is defined by the difference between the power of the output signal light when there is pump light and the power of the output signal light when there is no pump light. Non-Patent Documents 1, 2, and 3 describe methods for measuring Raman gain efficiency. Patent Document 1 discloses a method for measuring Raman gain by using pulsed light as Raman pump light.
JP 2004-317337 A NR Newbury, "Raman gain: pump-wavelength dependence in single mode-fiber," Opt. Lett. 27, 1232 (2002) CJS de Matos, KP Hansen and JR Taylor, "Experimental characterization of Raman gain efficiency of holey fiber," Electron. Lett. 39, 424 (2003) Z. Yusoff, JH Lee, W. Belardi, TM Monro, PC Teh and DJ Richardson, "Raman effects in a highly nonlinear holey fiber: amplification and modulation," Opt. Lett. 27, 424 (2002)

  The Raman gain measurement method for turning on / off the pump light requires either a long optical fiber ranging from several kilometers to several tens of kilometers and either high power pump light, or both. Both are difficult to obtain. In the conventional method, when the fiber to be measured is short or the pump power is insufficient, the Raman gain cannot be measured with high accuracy.

  It is an object of the present invention to provide a method and apparatus that can accurately measure Raman gain or Raman gain coefficient even with low-power pump light or a short fiber.

A Raman gain measurement method according to the present invention includes a probe light generation step for generating probe light having a probe wavelength intensity-modulated at a first frequency, a probe light supply step for supplying the probe light to a Raman amplification medium, A pump light generation step for generating pump light having a pump wavelength different from the probe wavelength, the pump light being intensity-modulated at a second frequency different from the first frequency; A pumping light supplying step for supplying to the optical fiber, a conversion step for converting the optical component of the probe wavelength output from the Raman amplification medium into an electric signal, and an amplitude for detecting the amplitude of the first frequency component from the electric signal. 1 detection step and a component of a frequency corresponding to either the sum or difference of the first frequency and the second frequency from the electric signal. The Raman amplification and a second detection step of detecting a width by the calculation of multiplying a predetermined coefficient that reflects the effective cross-sectional area of the first and second both amplitude ratio in the Raman amplification medium detected by the detecting step And a calculation step for calculating the Raman gain of the medium.

A Raman gain measuring apparatus according to the present invention includes a Raman amplification medium, a probe light source that generates probe light having a probe wavelength intensity-modulated at a first frequency, and supplies the probe light to the Raman amplification medium, and the first frequency. Is pump light intensity-modulated at a different second frequency and generates pump light having a pump wavelength different from the probe wavelength, and the pump light output from the pump light source is converted to the Raman amplification medium. An optical coupler that supplies the optical component of the probe wavelength output from the Raman amplification medium to an electrical signal, and a first detector that detects the amplitude of the first frequency component from the electrical signal. A detection device, and a second detection device that detects, from the electrical signal, an amplitude of a frequency component corresponding to either the sum or difference of the first frequency and the second frequency; By comprising an arithmetic unit by a calculation of multiplying a predetermined coefficient that reflects the effective cross-sectional area of the Raman amplification medium to the ratio of the output of the first and second detecting device for calculating the Raman gain of the Raman amplification medium Features.

  According to the present invention, the Raman gain can be measured with high accuracy even with a short sample of low pump power and / or Raman amplification medium.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. The tunable laser 10 is capable of continuous laser oscillation in a measurement wavelength range of Raman gain, for example, in the range of 1510 nm to 1650 nm, and outputs CW probe light or test light corresponding to signal light to be Raman amplified. The wavelength control device 12 sweeps the oscillation wavelength of the laser 10. The laser 14 generates CW laser light that serves as pump light for Raman amplification. In this embodiment, the laser oscillation wavelength of the laser 14 is 1490 nm. The oscillation wavelength of the laser 10 is expressed as λs, and the oscillation wavelength of the laser 14 is expressed as λp.

In this embodiment, probe light and pump light are intensity-modulated at different frequencies, and the Raman gain is measured using the beat component. By modulating the frequency f ₂ of the modulation frequency f ₁ and the pump light of the probe light is different, the probe light output from the Raman amplification medium, Raman amplified portion and Raman amplified not part emerges.

The intensity modulator 16 performs rectangular wave modulation on the CW probe light output from the laser 10 in accordance with the clock having the duty ratio of 50% from the clock generator 18 and the frequency f ₁ . Similarly, the intensity modulator 20 performs rectangular wave modulation on the CW pump light output from the laser 14 in accordance with a clock having a duty ratio of 50% and a frequency f ₂ from the clock generator 22. Clock generators 18 and 22 generate clocks with a duty ratio of 50% at frequencies f ₁ and f ₂ , respectively, according to the output frequency of oscillator 24. The intensity modulators 16 and 20 may be electrical or mechanical. As a mechanical intensity modulator, there is a so-called chopper. In an experiment for actually measuring the effect of this embodiment, a chopper was used as the intensity modulators 16 and 20, and f ₁ was set to 283 Hz and f ₂ was set to 340.6 Hz.

  In this embodiment, the intensity modulation by the intensity modulators 16 and 20 is synchronized, but it is not necessary to be synchronized in operation.

  The intensity-modulated probe light output from the intensity modulator 16 is incident on the Raman amplification medium 26. A WDM (wavelength division multiplexing) optical coupler 28 causes the intensity-modulated pump light output from the intensity modulator 20 to enter the Raman amplification medium 26 from the rear side. The pump light may be incident on the Raman amplification medium 26 from the front, that is, so as to propagate in the same direction as the propagation direction of the probe light. However, in the case of forward incidence, a WDM optical filter with a high extinction ratio for separating weak probe light from pump light is required.

  FIG. 2 shows a timing chart of the present embodiment. FIG. 2A shows an example of the waveform of pump light subjected to rectangular wave modulation. FIG. 2B shows an example of the waveform of the probe light subjected to rectangular wave modulation. FIG. 2C schematically shows the on / off change of Raman amplification by the pump light shown in FIG. FIG. 2D shows an example of the waveform of the probe light output from the Raman amplification medium 26. When the pump light is incident, the Raman amplification medium 26 exhibits a Raman amplification effect.

  The optical filter 30 removes the light having the pump wavelength λp from the output light of the Raman amplification medium 26. The optical filter 30 is composed of, for example, a broadband optical filter that transmits light having a wavelength of 1500 nm or more. The light receiver 32 converts the output light of the optical filter 30 into an electrical signal. If the influence of ASE (Amplified Spontaneous Emission) light is to be removed, the optical filter 30 may be an optical bandpass filter whose transmission center wavelength is controlled to a wavelength λs equal to the oscillation wavelength of the laser 10 by the wavelength control device 12. .

The electric bandpass filter 34 separates and extracts the component of the frequency f ₁ from the output electric signal of the light receiver 32, and the electric bandpass filter 36 extracts the component of the frequency (f ₁ + f ₂ ) from the output electric signal of the light receiver 32. Separate and extract. The arithmetic circuit 38 calculates the Raman gain coefficient g _R of the Raman amplifying medium 26 from the outputs of the filters 34 and 36 in accordance with mathematical expressions described later.

The principle of measuring the Raman gain coefficient g _{R according} to this embodiment will be described. The rectangular wave-modulated Raman pump light is P _pump (t), and its peak intensity is P _p . Also, let the rectangular wave modulated probe light be P _i (t), and its peak intensity be P _s . Expressed in Fourier series,
P _i (t) = P _s / 2 + (2P _s / π) sin (ω ₁ t) + (2P _s / 3π) sin (3ω ₁ T) + (1)
P _pump (t) = P _P / 2 + (2P _P / π) sin (ω ₂ t + φ) + (2P _P / 3π) sin (3ω ₂ T + 3φ) +… (2)
However, ω ₁ = 2πf ₁ and ω ₂ = 2πf ₂ .

When the fiber length of the Raman amplification medium 26 is short enough that the probe light and the pump light can pass through the Raman amplification medium 26 within one period of the modulation frequency, the output light P _o (t) of the Raman amplification medium 26 with the wavelength λs. Is represented by the following formula. That is,
P _o (t) = P _i (t) exp {g _R L _eff P _pump (t) / A _eff } (3)
Where A _eff is the effective cross-sectional area of the Raman amplification medium 26. The effective area A _eff can be separately measured as an optical transmission parameter of the Raman amplification medium 26 that is a sample. L _eff is the effective fiber length of the Raman amplification medium 26,
L _eff = (1 / α _P ) {1-exp (-α _P L)} (4)
Defined by α _P is the loss of the Raman amplification medium 26 at the pump wavelength λp, and L is the length of the Raman amplification medium 26.

Substituting Equations (1) and (2) into Equation (3). Assuming that the Raman gain coefficient g _R is sufficiently small,
P _o (t)
≒ P _i (t) {1 + g _R L _eff P _pump (t) / A _eff }
= P _P / 2 + P _s P _P g _R L _eff / (4A _eff ) + (2P _s / π) sin (ω ₁ t) + (P _s P _P g _R L _eff / πA _eff ) sin (ω ₂ t + φ)
+ (4P _s P _P g _R L _eff / π ² A _eff ) sin (ω ₁ t) sin (ω ₂ t + φ) + (5)
It becomes.

(4P _s P _P g _R L _eff / π ² A _eff ) sin (ω ₁ t) sin (ω ₂ t + φ) includes the angular frequency components ω ₁ + ω ₂ and ω ₁ −ω ₂ . Although expressed as a sum, the amplitude of any component is 2P _s P _P g _R L _eff / π ² A _eff .

In the embodiment shown in FIG. 1, the electrical filter 34 separates and extracts the component of the angular frequency ω ₁ , and the detector 35 detects the amplitude V ₁ of the output of the filter 34. The electrical filter 36 separates and extracts the component of the angular frequency ω ₁ + ω ₂ , and the detector 37 detects the amplitude V ₂ of the output of the filter 36. The ratio of V ₁ and _{V 2} is,
V ₁ / V ₂ = (2P _s / π) / {2P _s P _P g _R L _eff / π ² A _eff } (6)
After all,
g _R = (V ₂ / V ₁ ) (πA _eff / P _P L _eff ) (7)
It becomes. Since P _P and _{L eff} and _{A eff} is known, previously inputted to the arithmetic circuit 38. The arithmetic circuit 38 calculates the Raman gain coefficient g _R according to the above formula from the outputs V ₁ and V ₂ of the detectors 35 and 37 and P _P , L _eff and A _eff inputted in advance.

Since the Raman gain of the Raman amplifying medium 26 is g _R L _eff P _P , the arithmetic circuit 38 _{converts the} Raman gain g _R L _eff P _P to the following equation.
g _R L _eff P _P = πA _eff (V ₂ / V ₁ )
You may calculate according to.

By taking the ratio of the outputs V ₁ and V ₂ of the detectors 35 and 37, fluctuations in the power P _s of the probe light can be offset.

According to this embodiment, for example, the Raman gain can be measured even when the pump light power is about 5 mW. FIG. 3 shows an example of measurement. The horizontal axis represents the peak power P _P of the pumping light, and the vertical axis shows the measured Raman gain. A pump wavelength λp was 1490 nm, a probe light wavelength λs was 1590 nm, and a 50 m long dispersion compensating fiber was used as the Raman amplification medium 26. The modulation frequency of the probe light is 283 Hz, and the modulation frequency of the pump light is 340 Hz.

To detect the amplitude of the output electrical signal from the frequency _{f 1} of the component and the frequency _f 1 + _{f 2} components of the light receiver 32 has been described an example of using an electrical filter 34, 36 a detector 35 and 37, A lock-in amplifier may be used. FIG. 4 shows a schematic block diagram of such a modification. The same components as those in FIG. 1 are denoted by the same reference numerals.

In the embodiment shown in FIG. 4, the lock-in amplifier 40 detects the amplitude V ₁ of the component synchronized with the frequency f ₁ from the output electric signal of the light receiver 32, and the lock-in amplifier 42 outputs the output electric signal of the light receiver 32. detecting the amplitude _{V 2} of the component to be synchronized with the frequency _f 1 + _{f 2} from. In order to detect synchronization of the lock-in amplifier 40, the output clock of the clock generator 18 is applied to the synchronization signal input terminal of the lock-in amplifier 40. Further, the mixer 44 multiplies the output clocks of the clock generators 18 and 22 to generate a component of frequency f ₁ + f ₂ . An electric bandpass filter 46 with a center frequency f ₁ + f ₂ extracts a component with a frequency f ₁ + f ₂ from the output of the mixer 44 and applies it to the synchronization signal input terminal 42 of the lock-in amplifier 42. The lock-in amplifiers 40 and 42 appropriately adjust the phase of the input synchronization signal. Thereby, the amplitudes of the components of the frequencies f ₁ and f ₁ + f ₂ can be detected.

  FIG. 5 shows a schematic configuration diagram of an embodiment in which a mechanical chopper is used for intensity modulation of probe light and pump light. The same components as those in the embodiment shown in FIGS. 1 and 4 are denoted by the same reference numerals.

  The chopper 50 includes a chopper plate 52 and a motor 54 that rotates the chopper plate 52 at a constant speed. A plan view of the chopper plate 52 is shown in FIG. The chopper plate 52 has six openings 52a evenly on the outer peripheral side and five openings 52b evenly on the inner peripheral side. The chopper plate 52 is disposed at a position where the opening 52 a transmits the probe light from the laser diode 10 and the opening 52 b transmits the pump light from the laser 14. The chopper control circuit 56 rotates the motor 54 at a constant speed according to the output of the photocoupler 58 that detects the rotation of the chopper plate 52. A rotation detection device other than the photocoupler 58 may be used. A rotation detection device such as the photocoupler 58 and rotation control by its output are well known.

By the chopper 50, the probe light is subjected to rectangular wave modulation at a frequency of 283 Hz (= f ₁ ), and the pump light is subjected to rectangular wave modulation at a frequency of 340 Hz (f ₂ ).

The chopper control circuit 56 applies a clock having a frequency f ₁ synchronized with the rotation of the motor 54 to the lock-in amplifier 40, and applies a clock having a frequency f ₁ + f ₂ synchronized with the rotation of the motor 54 to the lock-in amplifier 42.
(Other)

Although two methods for detecting the amplitudes of the components of the frequencies f ₁ and f ₁ + f ₂ have been described, it is obvious that other methods may be adopted.

  Although the case where the probe light and the pump light are subjected to rectangular wave modulation has been described, the waveform of the modulated wave may have other shapes. Correction corresponding to the waveform may be incorporated in the arithmetic circuit 38 or applied to the output of the arithmetic circuit 38.

  Although the invention has been described with reference to specific illustrative embodiments, various modifications and alterations may be made to the above-described embodiments without departing from the scope of the invention as defined in the claims. This is obvious to an engineer in the field to which the present invention belongs, and such changes and modifications are also included in the technical scope of the present invention.

It is a schematic block diagram of the first embodiment of the present invention. It is a timing chart which shows the example of a waveform of a present Example. It is an actual measurement example of pump light power versus Raman gain. It is a schematic block diagram of the second embodiment of the present invention. It is a schematic block diagram of the third embodiment of the present invention. 3 is a plan view of a chopper plate 52. FIG.

Explanation of symbols

10: wavelength tunable laser 12: wavelength control device 14: pump laser 16: intensity modulator 18: clock generator 20: intensity modulator 22: clock generator 24: oscillator 26: Raman amplification medium 28: WDM optical coupler 30: light Filter 32: Light receiver 34, 36: Electric band pass filter 35, 37: Detector 38: Arithmetic circuit 40, 42: Lock-in amplifier 44: Mixer 46: Electric band pass filter 50: Chopper 52: Chopper plate 54: Motor 56: Chopper control circuit 58: Photocoupler

Claims (17)

A probe light generation step for generating probe light having a probe wavelength intensity-modulated at a first frequency;
A probe light supplying step for supplying the probe light to the Raman amplification medium (26);
A pump light generation step of generating pump light having a pump wavelength different from the probe wavelength, the pump light being intensity-modulated at a second frequency different from the first frequency;
A pump light supply step for supplying the pump light to the Raman amplification medium (26);
A conversion step of converting an optical component of the probe wavelength output from the Raman amplification medium (26) into an electrical signal;
A first detection step of detecting the amplitude of the first frequency component from the electrical signal;
A second detection step of detecting an amplitude of a frequency component corresponding to either the sum or difference of the first frequency and the second frequency from the electrical signal;
A calculation step of calculating a Raman gain of the Raman amplification medium by an operation of multiplying a ratio of both amplitudes detected in the first and second detection steps by a predetermined coefficient reflecting the effective area of the Raman amplification medium. A Raman gain measuring method comprising: The first detection step includes a first separation step of separating the component of the first frequency from the electric signal, and a step of detecting the amplitude of the component separated in the first separation step.
The second detection step separates a frequency component corresponding to either the sum or difference of the first frequency and the second frequency from the electrical signal, and the second separation step, 2. The Raman gain measuring method according to claim 1, further comprising a step of detecting an amplitude of the component separated in the separation step. The first detection step includes a step of synchronously detecting the component of the first frequency,
The second detection step is any one of a frequency component corresponding to the sum of the first frequency and the first frequency, and a frequency component corresponding to a difference between the first frequency and the first frequency. The method of claim 1, further comprising the step of synchronously detecting the frequency component of The probe light generation step includes a step of generating CW probe light having the probe wavelength and a probe light intensity modulation step of modulating the intensity of the CW probe light. The Raman gain measuring method according to the item.   5. The Raman gain measuring method according to claim 4, wherein the probe light intensity modulating step performs rectangular wave modulation on the CW probe light. 6. The pump light generation step according to claim 1, wherein the pump light generation step includes a step of generating CW pump light of the pump wavelength and a pump light intensity modulation step of modulating the intensity of the CW pump light. The Raman gain measuring method according to the item.   The Raman gain measurement method according to claim 6, wherein the pump light intensity modulation step performs rectangular wave modulation on the CW pump light.   The calculation step includes a Raman gain coefficient of the Raman gain from a ratio of both amplitudes detected in the first and second detection steps, an effective cross-sectional area and effective length of the Raman amplification medium, and a power of the pump light. The Raman gain measuring method according to any one of claims 1 to 6, wherein the Raman gain is calculated. A Raman amplification medium (26);
A probe light source (10, 16) for generating probe light having a probe wavelength intensity-modulated at a first frequency and supplying the probe light to the Raman amplification medium;
A pump light source (14, 20) that generates pump light having a pump wavelength different from the probe wavelength, the pump light being intensity-modulated at a second frequency different from the first frequency;
An optical coupler (28) for supplying the pump light output from the pump light source to the Raman amplification medium (26);
A light receiver (32) for converting the optical component of the probe wavelength output from the Raman amplification medium (26) into an electrical signal;
A first detector (34, 40) for detecting the amplitude of the first frequency component from the electrical signal;
A second detection device (36, 42) for detecting an amplitude of a component of a frequency corresponding to either the sum or difference of the first frequency and the second frequency from the electrical signal;
An arithmetic unit (38) for calculating a Raman gain of the Raman amplifying medium by an operation of multiplying a ratio of the outputs of the first and second detecting devices by a predetermined coefficient reflecting the effective area of the Raman amplifying medium.
A Raman gain measuring apparatus comprising: A first electric filter (34) for separating the first frequency component from the electric signal, and a first detector for detecting the amplitude of the output of the first electric filter. And (35),
A second electrical filter (36) for separating a frequency component corresponding to either the sum or difference of the first frequency and the second frequency from the electrical signal; The Raman gain measuring apparatus according to claim 9, further comprising: a second detector that detects an amplitude of an output of the second electric filter; The first detection device includes a first synchronization detector (40) that synchronously detects a component of the first frequency from the electrical signal,
The second detection device includes a second synchronization detector (42) for synchronously detecting a frequency component corresponding to either the sum or difference of the first frequency and the second frequency. 10. The Raman gain measuring apparatus according to claim 9, wherein   12. The Raman gain measuring apparatus according to claim 11, wherein each of the first and second synchronization detectors includes a lock-in amplifier. The probe light source comprises a probe laser (10) that generates CW probe light of the probe wavelength and a first light intensity modulator (16) that modulates the intensity of the CW probe light. The Raman gain measuring apparatus according to any one of 9 to 12.   The Raman gain measuring apparatus according to claim 13, wherein the first light intensity modulator performs rectangular wave modulation on the CW probe light. The pump light source includes a pump laser (14) that generates CW pump light having the pump wavelength and a second light intensity modulator (20) that modulates the intensity of the CW pump light. The Raman gain measuring apparatus according to any one of 9 to 14.   The Raman gain measurement apparatus according to claim 15, wherein the second light intensity modulator performs rectangular wave modulation on the CW pump light.   The arithmetic unit calculates the Raman gain coefficient of the Raman gain from the ratio of both amplitudes detected by the first and second detection devices, the effective cross-sectional area and effective length of the Raman amplification medium, and the power of the pump light. The Raman gain measuring device according to claim 9, wherein the Raman gain measuring device according to claim 9 is calculated.

Images