JPH04268442A - Cavity measuring method and apparatus - Google Patents

Abstract

PURPOSE:To enable the measurement of an optical path length and an effective reflection factor of a cavity to be conducted even when they are severable mm and below 1% respectively. CONSTITUTION:Current having a DC bias current Iop overlapping a modulation current In is supplied to a semiconductor laser 2 and a radiation light thereof is detected with a photo detector 3 to measure a harmonic distortion rate D of an output signal of the photo detector 3. A temperature change T corresponding to the cycle of the distortion rate D is measured by varying the temperature T of the semiconductor laser 2 to determine a wavelength change lambdaof a laser light corresponding to the temperature change T. Thus, an optical path length L and an effective reflection factor R of a cavity 10 arranged in an optical path of the laser light are calculated based on the distortion rate D and the wavelength change lambda.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cavity measuring method and apparatus for measuring the optical path length and effective reflectance of a cavity existing in the optical path of an optical system.

0002

2. Description of the Related Art For example, in analog optical transmission, if a cavity is carelessly placed in an optical path, the waveform will be distorted and harmonics will be generated, which may interfere with communications at other frequencies. Therefore, it is important to check whether an undesirable cavity exists in the optical path and, if so, to examine the optical characteristics of the cavity.

FIG. 9 shows the conventional cavity measurement principle. In the same figure (A), the cavity 10 is, for example, a glass plate. When pulsed light as shown in FIG. 2B is emitted from the pulsed light source 12 and applied perpendicularly to the cavity 10 via the half mirror 14, the end face 10 of the cavity 10
Part of it is reflected by a and 10b, further reflected by a half mirror 14, and detected by a photodetector 16. The output of the photodetector 16 is as shown in FIG.
By measuring the time difference Δt between the pulsed light reflected at a and the pulsed light reflected at the end surface 10b, the cavity 1
The optical path length L between the end faces of 0 can be obtained. Also,
By measuring the ratio between the intensity of the pulsed light emitted from the pulsed light source 12 and the intensity of the pulsed light detected by the photodetector 16, the reflectance at the end faces 10a and 10b can be determined.

0004

However, when the optical path length L of the cavity 10 is several mm, the time difference Δt becomes extremely small.On the other hand, when the reflectance at the end faces 10a and 10b is 1% or less, Such measurements have been difficult because it is necessary to widen the width of the pulsed light output from the pulsed light source 12 to the extent that it can be detected. That is, if the time width of the pulsed light is TW and the luminous flux is c, then the end face 1
In order to distinguish between the pulsed light reflected at 0a and the pulsed light reflected at the end face 10b, it is necessary to set 2L/c>TW. For example, when L=5mm, TW<
Since it required 33 ps, optical detection was difficult. In the conventional cavity measuring device, the end faces 10a and 10
When the reflectance of b was 1% or less, the optical path length L and reflectance of the cavity 10 could not be measured unless the optical path length L between the end faces was at least several centimeters or more.

In view of these problems, the object of the present invention is to improve the optical path length and effective An object of the present invention is to provide a cavity measuring method and device that can measure reflectance.

[0006]

[Means for Solving the Problems and Their Effects] FIG. 1 shows the principle configuration of a cavity measuring device according to the present invention. This cavity measuring device includes the following components.

1 is a current source, and a DC bias current Io
A current obtained by superimposing p and modulation current Im is output. A semiconductor laser 2 is supplied with the output of the current source 1 and emits laser light. 3 is a photodetector that detects the laser beam. 4 is a spectrum analyzer, and a photodetector 3
Measure the spectral distribution of the output signal. 4 is a distortion factor calculation means, which calculates a harmonic distortion factor based on the output of the spectrum analyzer 4. Reference numeral 5 denotes a temperature control means, which changes the temperature T of the semiconductor laser 2. Reference numeral 6 denotes a period measuring means, which measures a temperature change ΔT corresponding to the period (including a plurality of periods) of the strain rate D when the temperature T is changed. Reference numeral 7 denotes a conversion means, which essentially obtains a wavelength change Δλ of the laser beam corresponding to a temperature change ΔT. Substantially means that instead of the wavelength change Δλ, a corresponding wave number change or angular frequency change is determined. 8 is a cavity optical path length calculating means, which calculates the optical path length L of the cavity 10 existing in the optical path of the laser beam based on the wavelength change Δλ. Reference numeral 9 denotes a reflectance calculating means, which calculates the effective reflectance R of the cavity 10 existing in the optical path of the laser beam by substantially calculating the optical path length L.
and the distortion rate D. In this case, "substantially" includes the use of a related wavelength change Δλ instead of the optical path length L.

Furthermore, in the cavity measurement method according to the present invention, a current in which a DC bias current Iop and a modulation current Im are superimposed is supplied to the semiconductor laser 2, and the light emitted from the semiconductor laser 2 is detected by the photodetector 3. Then, the harmonic distortion rate D of the output signal of the photodetector 3 is measured, and the temperature T of the semiconductor laser 2 is measured.
The temperature change ΔT corresponding to the period of the strain rate D is measured by changing the temperature change ΔT, and the wavelength change Δλ of the laser light corresponding to the temperature change ΔT is measured.
is substantially determined, and the optical path length L and effective reflectance R of the cavity 10 existing in the optical path of the laser beam are calculated based on the distortion factor D and the wavelength change Δλ.

The operation of the cavity measuring method and apparatus described above is as follows. Note that, regarding the distortion rate, only the second-order distortion rate D2, which has the highest sensitivity, will be considered. This secondary distortion factor D2 is expressed by the following equation.

D2=Ncso(mR)2×{(2πη(Iop−
Ith) τ}4×{cos(ωτ)}2×{exp(-
πΔντ)}2
...(1) Here, Ncso: When the modulation current Im is a superimposition of multiple sinusoidal currents, it is the number of combinations of secondary intermodulation distortion, and when there is one sinusoidal current, it is 1m
: Optical modulation degree Pm0/Pop of the modulation signal (Fig. 3) η: FM modulation efficiency (Hz/mA) of the semiconductor laser 20 Iop: DC bias current supplied to the semiconductor laser 20 (laser operating point current) Ith: semiconductor laser 20 Threshold current τ: Cavity round trip time, where c is the speed of light, τ=2L/c ω: Center angular frequency Δν of the laser beam: Spectral line width (full width at half maximum) of the laser beam.

Normally, Δν=1 to 50 MHz, and when L=1 to 100 mm, τ=5 to 500 ps. Therefore, in the above equation (1), exp(-πΔ
ντ)≒1, and the following equation approximately holds true.

D2=Ncso×(mR)2×{(2πη(Iop
−Ith)τ}4×{cos(ωτ)}2

・・・
(2) If the secondary distortion factor D2 changes by one period when the angular frequency ω is changed by Δω, then the following equation holds true.

[0013]Δω×τ=π

(3) When τ=2L/c is substituted into this equation (3), the optical path length L is expressed by the following equation.

L=πc/(2Δω)
・・・
・(4) If the center wavelength of the laser beam is λ, then λ=2πc
/ω, Δλ=2πcΔω/ω2, so equation (4) can be expressed as the following equation.

L=λ2/(4Δλ)
・
(5) Here, the center wavelength λ of the output light of the semiconductor laser 2 changes as shown in FIG. 7, for example, when the temperature T of the semiconductor laser 2 is changed. Therefore, the semiconductor laser 2
The second-order distortion factor D2 is measured while changing the temperature T of the semiconductor laser 2, and the temperature change ΔT of the semiconductor laser 2 corresponding to one cycle change of the second-order distortion factor D2 is measured, and the wavelength change Δλ corresponding to this temperature change ΔT is calculated. Once determined, the optical path length L can be determined from the above equation (5).

Once the optical path length L is determined, τ=2L/c is determined, and furthermore, the effective reflectance R is determined from the above equation (2).

According to the present invention, as shown in the test examples described later, even if the cavity 10 has an optical path length L between opposing end surfaces of several mm and an effective reflectance R of these end surfaces of 1% or less, the cavity 10 It becomes possible to measure these optical path length L and effective reflectance R.

The effective reflectance R is calculated using, for example, the maximum value Dm of the distortion factor D obtained when the laser temperature T is changed.

With this configuration, the measurement accuracy of the effective reflectance R is improved. For example, in the case of D=D2, if the maximum value of D is D2m, the following equation holds from the above equation (2).

R2=D2m/[Ncso×m2×{(2πη(I
op-Ith)τ}4]...(6) The above modulation current I
For example, m is obtained by superimposing a plurality of sinusoidal currents having mutually different frequencies f in order to increase the intermodulation distortion factor.

In the case of this configuration, the measurement sensitivity is increased, and the measurement accuracy of the effective reflectance R and the optical path length L of the cavity 10 is improved.

[0022]

Embodiments Hereinafter, embodiments of the present invention will be explained based on the drawings.

(1) First embodiment FIG. 2 shows the configuration of the cavity measuring device.

The semiconductor laser 20 is supplied with an operating point current Iop larger than its threshold current Ith from a DC bias power supply 22. This Iop is supplied with a modulation current Im0s from a sinusoidal modulation signal source 24 via a capacitor 26.
in2πft are superimposed. Therefore, the current I supplied to the semiconductor laser 20 and the optical output P of the semiconductor laser 20 are as shown in FIG.

The laser light emitted from the semiconductor laser 20 is collimated through the collimator 28, passes through the cavity 10 via the optical isolator 30, and is detected by the photodetector 32. Since the characteristics of the semiconductor laser 20 change when the reflected light from the cavity 10 enters the semiconductor laser 20, an optical isolator 30 is used to prevent this. The laser light emitted from the optical isolator 30 may be passed through an optical fiber (not shown).

The output of the photodetector 32 is sent to a high frequency amplifier 34.
After being amplified, the signal is supplied to a spectrum analyzer 36, and a spectral distribution as shown in FIG. 4 is measured. The detection light contains harmonics that are integral multiples of the modulation frequency f because the direct light L1 transmitted through the cavity 10 and the delayed light L2 reflected at the rear end face 10a that was reflected at the end face 10b of the cavity 10 interfere. This is because the waveform is distorted.

When the wavelength λ of the laser beam is constant, this waveform distortion is determined by the effective reflectance R of the end faces 10a and 10b and the optical path length L between the end faces 10a and 10b. The effective reflectance R is the reflectance of the end surface 10a, R1, and the end surface 10b.
When the reflectance of is R2, it is expressed as (R1R2)1/2. When the wavelength λ of the laser beam changes, the amount of waveform distortion changes according to the above equation (1). On the other hand, the wavelength λ of the laser light is
It changes when the temperature of the semiconductor laser 20 is changed. Therefore,
It has the following configuration.

A Peltier element 38 that generates or absorbs heat due to the Peltier effect is arranged near the semiconductor laser 20, and a current is supplied to the Peltier element 38 from the temperature controller 40. Furthermore, in order to detect the temperature of the semiconductor laser 20,
A temperature detector 42 is arranged near the semiconductor laser 20. The output of the temperature detector 42 is amplified through a DC amplifier 44, then converted into a digital value by an A/D converter 46, fed back to the temperature controller 40, and also supplied to the microcomputer 48. The temperature regulator 40 controls the current supplied to the Peltier element 38 so that the detected temperature approaches the set temperature supplied from the microcomputer 48.

The microcomputer 48 is supplied with constants necessary for determining the optical characteristic constants L and R of the cavity 10 from a keyboard 50. The microcomputer 48 includes a spectrum analyzer 36, an A/D
Based on the data supplied from the transducer 46 and the keyboard 50, the optical characteristic constants L and R of the cavity 10 are determined and the results are supplied to the display 52 for display.

Next, the software configuration of the microcomputer 48 will be explained based on FIG. 5. In the following,
The numbers in parentheses are step identification numbers in FIG.

(100) Operate the keyboard 50 to obtain the following constants: m: optical modulation degree Pm0/Pop of the modulation signal (FIG. 3) η: FM modulation efficiency (Hz/mA) of the semiconductor laser 20 Iop:
DC bias current (laser operating point current) supplied to the semiconductor laser 20 Ith: Threshold current of the semiconductor laser 20 λ: Center wavelength of the laser beam α: Rate of change of the wavelength λ with respect to the temperature T of the semiconductor laser 20 (see FIG. 7) ) is read into the microcomputer 48.

(102) Turn on the DC bias power supply 22 and the sine wave modulation signal source 24 to cause the semiconductor laser 20 to emit laser light.

(104) The initial set temperature of the semiconductor laser 20 is supplied to the temperature controller 40. When the detected temperature from the A/D converter 46 matches the set temperature and becomes stable, the set temperature supplied to the temperature regulator 40 is gradually raised or lowered to change the temperature of the semiconductor laser 20. The center wavelength λ of the laser light emitted from the semiconductor laser 20 is changed. In parallel with such operations, the following steps 106 and 108 are performed.

(106) Local maximum value D2m of second-order distortion factor D2,
and a temperature change Δ corresponding to one cycle change in the secondary strain rate D2.
Detect T.

(108) Calculate the change in wavelength λ corresponding to this temperature change ΔT, Δλ=αΔT.

(110) Calculate the optical path length L using the above equation (5).

(112) Calculate the effective reflectance R using the above equation (6).

In the manner described above, the optical path length L and effective reflectance R of the cavity 10 can be easily determined.

(2) Second embodiment FIG. 6 shows the configuration of a cavity measuring device according to the second embodiment.

In this cavity measuring device, the second-order distortion factor D
In order to increase measurement sensitivity by increasing 2, a plurality of sinusoidal currents having different angular frequencies are superimposed and outputted from the modulation signal source 24A. That is, modulation signal source 2
4A consists of 24k (k=0 to n) that outputs a sine wave current of Im0sin(2πf+kβ)t, and a mixer 25 that superimposes these outputs.

Furthermore, an optical attenuator 54 is arranged between the cavity 10 and the photodetector 32 in order to prevent the output of the photodetector 32 from being saturated. moreover,
A bandpass filter 56 is connected between the high frequency amplifier 34 and the spectrum analyzer 36 to prevent saturation of the output of the spectrum analyzer 36, which allows only specific harmonics to pass.

The other points are the same as the first embodiment.

(3) Test example The test was conducted using the cavity measuring device shown in FIG. The test conditions are as follows.

Semiconductor laser 20: One with a second-order intermodulation distortion factor of -60 to -70 dB. Wavelength λ=
1.55 μm (ω = 1.22 × 1015) Spectral linewidth (full width at half maximum) of laser light Δν = 30 MHz Threshold current Ith = 15 mA FM modulation efficiency η = 300 MHz/mA α: 0.0988 nm/°C (Figure 7 ) DC bias power supply 22: One that does not generate large noise around 54MHz Operating point current Iop = 50mA Modulation signal source 24A: Approximately 6.615M between 55.25 and 313.25MHz
Outputs 40 channels of sine wave signals equally spaced in Hz (n=3
9) The amplitude of each sine wave current is the same, and the degree of optical modulation m = 0.032 for each sine wave 2nd order intermodulation distortion rate -70 dB or less Temperature controller 40: A device with a setting accuracy of 0.1°C or less Bandpass filter 56: Passes only signals around 54MHz (The frequency at which the second-order intermodulation distortion rate is measured is 54MHz,
In this case, the number of secondary intermodulation distortion combinations NCSO = 29)
FIG. 8 is a graph showing actually measured values of the relationship between temperature T and second-order intermodulation distortion rate D2 under these conditions. In this case, ΔT
=1.3℃, Δλ=0.13nm, D2m=-44dB
Therefore, L=4.5 mm and R=-27 dB were obtained. This result was in good agreement with the values of L and R measured directly by other methods.

[0045]

As explained above, according to the cavity measuring method and apparatus of the present invention, the optical path length L between the opposing end surfaces is several mm, and the effective reflectance R of these end surfaces is 1% or less. Also, these optical path lengths L of the cavity
and the effective reflectance R can be measured, thus:
This provides an excellent effect in that it is possible to know whether or not such an undesirable short cavity is carelessly disposed in the optical path of various optical systems, and to what extent it is undesirable.

If the effective reflectance of the cavity is calculated using the maximum value of the strain rate obtained when the laser temperature is changed, there is an effect that the measurement accuracy of the effective reflectance is improved. Furthermore, if the modulation current is made by superimposing multiple sinusoidal currents with different frequencies and the intermodulation distortion factor is increased, the measurement accuracy of the effective reflectance R and optical path length L of the cavity will be improved. play.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the principle configuration of a cavity measuring device according to the present invention.

FIG. 2 is a configuration diagram of a cavity measuring device according to the first embodiment.

FIG. 3 is a modulation input/output characteristic diagram of a semiconductor laser.

FIG. 4 is a detection signal spectrum distribution diagram obtained by a spectrum analyzer.

FIG. 5 is a flowchart of a cavity measurement procedure showing the software configuration of the microcomputer.

FIG. 6 is a configuration diagram of a cavity measuring device according to a second embodiment.

FIG. 7 is a graph showing actual measured values of changes in wavelength with respect to temperature of a semiconductor laser.

FIG. 8 is a graph showing actual measured values of changes in strain rate with respect to temperature of a semiconductor laser.

FIG. 9 is a diagram illustrating the principle of conventional cavity measurement.

[Explanation of symbols]

10 Cavities 10a, 10b End face 20 Semiconductor laser 22 DC bias power supply 24, 240 to 24n Sine wave modulation signal source 24A
Modulation signal source 28 Collimator 30 Optical isolator 32 Photodetector 34 High frequency amplifier 36 Spectrum analyzer 38 Peltier element 25 Mixer 54 Optical attenuator 56 Bandpass filter

Claims (4)

[Claims] 1. A current in which a DC bias current (Iop) and a modulation current (Im) are superimposed is supplied to a semiconductor laser (2), and the light emitted from the semiconductor laser is transmitted to a photodetector (3).
), and the harmonic distortion rate (D) of the output signal of the photodetector
is measured, the temperature (T) of the semiconductor laser is changed, the temperature change (ΔT) corresponding to the period of the strain rate is measured, and the wavelength change (Δλ) of the laser light corresponding to the temperature change is substantially calculated. cavity (1) existing in the optical path of the laser beam.
A cavity measuring method characterized in that the optical path length (L) and effective reflectance (R) of 0) are calculated based on the distortion rate and the wavelength change. 2. The effective reflectance (R) is determined by the distortion rate (D) obtained when the laser temperature (T) is changed.
2. The cavity measuring method according to claim 1, wherein the calculation is performed using a maximum value (Dm) of . 3. The modulation current (Im) is characterized in that a plurality of sinusoidal currents having different frequencies (f) are superimposed to increase the intermodulation distortion rate (D). The cavity measuring method according to item 1 or 2. 4. A current source (1) that outputs a current obtained by superimposing a DC bias current (Iop) and a modulation current (Im), and a semiconductor laser (2) that is supplied with the output of the current source and emits laser light. ) and a photodetector (3) that detects the laser beam.
), a spectrum analyzer (4) that measures the spectral distribution of the output signal of the photodetector, and a distortion factor calculation means (4) that calculates a harmonic distortion factor (D) based on the output of the spectrum analyzer. a temperature control means (5) for changing the temperature (T) of the semiconductor laser; and a period measuring means (6) for measuring a temperature change (ΔT) corresponding to the period of the strain rate when the temperature is changed. , conversion means (7
), and a cavity (10
) for calculating the optical path length (L) of the laser beam based on the wavelength change; and reflectance calculation means (9) that calculates based on the distortion factor.
) A cavity measuring device characterized by having the following.