JPH02291940A - Apparatus for measuring reflectivity at edge of optical waveguide - Google Patents

Abstract

PURPOSE:To make it possible to measure the wavelength dependence of the reflectivity of the edge face of an optical waveguide by making reference light interfere with the reflected light from the edge face, and performing inverse Fourier transformation of said interference pattern. CONSTITUTION:As means for inputting the emitted light from a light source 1 into 4 to be measured a condenser lens 2, a 3-dB fiber coupler 3 and refractive-index com pensating liquid 5 are provided. As means for measuring the reflected light which is reflected from the edge face of the wave guide 4, a collimate lens 8, a photodetector 9, a waveform memory 10 and an operating device 11 are provided. As a means for splitting reference light from the emitted light from the light source 1 and a means for synthesizing the reference light with the reflected light, the coupler 3 is used. As means for changing the relative lengths of the light paths of the reference light and the reflected light within a range wherein the reference light and the reflected light are made to interfere, a collimate lens 6 and a total reflecting mirror 7 are provided. A means which performs inverse Fourier transformation of the interferogram generated by the relative change in the lengths of the optical paths is included in the operating device 11.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is utilized for forming an antireflection coating on the end face of an optical waveguide. In particular, it relates to measuring the reflectance of an end face on which an antireflection coating is formed.

The present invention measures the wavelength dependence of the reflectance of an optical waveguide end face with high precision by interfering a reference light with light reflected from an end face and subjecting this interference pattern to inverse Fourier transformation.

[Conventional technology]

Antireflection coatings have conventionally been used to prevent reflected light from entering semiconductor lasers and other elements due to reflection at the end faces of optical waveguides. In particular, the reflectance to the semiconductor laser is
It is necessary to suppress it to 10-6 or less.

FIG. 5 shows the configuration of a conventional optical waveguide end face reflectance measuring apparatus.

For example, a semiconductor laser with an oscillation wavelength of 1.3 μm is used as the light source 1, and the emitted light from the source 1 is incident on a 3 dB fiber coupler 3 via a condensing lens 2.

The 3 dB fiber coupler 3 divides the incident light from the light source 1 into two parts, one of which is emitted to the outside from the end face A, and the other is made to enter the optical waveguide to be measured vJI4 from the end face B. The end face A is cut diagonally, and the reflectance here is suppressed to 10-7 or less. The space between the end surface B and the incident end C of the optical waveguide 4 to be measured is immersed in a refractive index compensating liquid 5 to prevent reflection.

Light reflected at the output end D of the optical waveguide 4 to be measured {well, end surface B
The light then enters the 3 dB fiber coupler 3 again and is emitted from the end face E. The optical power meter 50 measures the power of the light emitted from this end surface E. Thereby, the reflectance of the output end D can be measured.

Optical waveguide to be measured 'a! When 4 is a glass waveguide with a refractive index of 1.46 and its output end D is cut smoothly, the end face reflectance at this output end D is 4%.

When a multilayer film for antireflection is deposited on the output end D, the reflectance becomes smaller than 4%.

[Problem to be solved by the invention] However, since the refractive index of the 3 dB fiber coupler and the refractive index of the optical waveguide to be measured are usually different, it is necessary to suppress reflection at the end face and the input end between them using a refractive index compensating liquid. It is difficult. Furthermore, even if the optical waveguide to be measured is a silica glass waveguide and its refractive index is very close to the refractive index of a 3 dB fiber coupler, the refractive index will change due to the condition or polishing of the end face, and in reality, the optical waveguide to be measured is 10- at the incident end of
3 reflections occur. For this reason, the limit for measuring the reflectance at the output end of the optical waveguide to be measured is about 10-3, and it has been difficult to measure reflectances lower than this.

In other words, the dynamic range was less than 30 dB L.

Furthermore, since the reflectance of the multilayer antireflection coating has a sharp wavelength dependence, it is necessary to find the wavelength at which the reflectance is minimum. However, when laser light is used, the measurable reflectance is limited to values for specific wavelengths.

It is also conceivable to use white light as a light source to measure the wavelength dependence of reflectance. However, when white light is spectrally analyzed, its optical power becomes very weak, making it difficult to measure a reflectance of approximately 1O-3 using this light.

An object of the present invention is to solve the above problems and provide an optical waveguide end face reflectance measurement device that can measure the end face reflectance of an optical waveguide to be measured without being affected by reflections at other points in the measurement system. do.

[Means for solving problems]

The optical waveguide end face reflection array measuring device of the present invention includes a light source, a means for inputting the emitted light from the light source into the optical waveguide to be measured, and a measuring device for measuring the reflected light reflected by the end face of the optical waveguide to be measured and appearing at the input end. An optical waveguide end face reflectance measuring device comprising means for branching a reference light from light emitted from a light source, means for multiplexing this reference light into reflected light, and means for causing this reference light to interfere with the reflected light. means for changing the relative optical path length of the reference light and the reflected light within a range, and the measuring means performs an inverse Fourier transform on an interference pattern (interferogram) caused by the change in the relative optical path length. It is characterized by including means.

Here, the interference pattern to be subjected to inverse Fourier transform, that is, the interferogram, is the interference intensity when the light emitted from the light source is divided into two, an optical delay is given to one half, and these two lights are combined. refers to the fringe that occurs within the

As the light source, one with a wide spectrum width, such as a light emitting diode, is used.

It is desirable to further include monitoring means for measuring the relative optical path length difference between the reference light and the reflected light. Such a monitoring means includes a reference light source whose emission spectrum is sufficiently different from that of the light source and whose emission spectrum width is sufficiently narrow, and a means for multiplexing the emitted light of this reference light source with the emitted light of the above-mentioned light source. , a phase modulation means for applying phase modulation of frequency f to the reference light or reflected light, and receiving the combined light obtained by the above-mentioned combining means, and a frequency f component and a frequency 2f component included in the received light output. and means for measuring the phase relationship between the two.

At this time, a frequency component is included in the interference pattern due to phase modulation. In order to eliminate this, it is desirable to include means for phase-detecting the interference signal obtained by receiving the combined light of the reference light and the reflected light.

The branching means and the combining means include a common fiber coupler, and the means for changing the optical path length is a total reflection mirror that makes the output light from one output end of the fiber coupler enter the fiber coupler again. and a moving means for moving the total reflection mirror in the beam direction.

As the phase modulation means, a phase modulator having a structure in which one arm of a fiber coupler is wound around a cylindrical electrostrictive vibrator, or a phase modulator having a structure in which a total reflection mirror is slightly vibrated can be used.

[For production]

The coherence length of light emitted from a light source is generally shorter than the length of the optical waveguide to be measured. Therefore, by measuring the interference pattern, it is possible to measure the reflectance in a short range, and it is possible to prevent influences from other parts.

Further, by using a light source with a wide spectrum width and performing inverse Fourier transform on the interference pattern, the wavelength dependence of the reflectance can be determined.

〔Example〕

FIG. 1 shows the configuration of an optical waveguide end face reflectance measuring device according to a first embodiment of the present invention.

This device is equipped with a light source 1, and a condenser lens 2 with a 3 dB
It is equipped with a fiber coupler 3 and a refractive index compensating liquid 5, and includes a collimating lens 8, a photodetector 9, a waveform memory 10, and an arithmetic unit as a means for measuring the reflected light reflected by the end face of the optical waveguide 4 to be measured and appearing at the input end. 11.

Here, the feature of this embodiment is that a 3 dB fiber coupler 3 is used as a means for branching the reference light from the light emitted from the light source 1 and a means for combining the reference light with the reflected light, so that the reference light is A collimating lens 6 and a total reflection mirror 7 are provided as means for changing the relative optical path length between the reference light and the reflected light within a range where the reference light interferes with the light, and the arithmetic unit 11 detects the change in the relative optical path length caused by the change in the relative optical path length. The method includes means for inverse Fourier transforming the interferogram.

Light source 1 has a spectral width Δλ = 600 people, a center wavelength λ,
= 1.3 μm, and its emitted light enters a 3 dB fiber coupler 3 through a condenser lens 2.

The 3 dB fiber coupler 3 divides this incident light into two, and outputs one of the two from the end face A. The light emitted from end face A is
It becomes a parallel beam by the collimating lens 6, is reflected by the total reflection mirror 7, and enters the 3 dB fiber coupler 3 again. This light becomes the reference light.

The other light split into two by the 3dB fiber coupler 3 is
The light enters the optical waveguide 4 to be measured from the end face B.

At this time, reflection occurs at the output end D of the optical waveguide 4 to be measured,
The reflected light enters the 3 dB fiber coupler 3 again.

The area between the end face B and the input end C of the optical waveguide 4 to be measured is immersed in the interference compensating liquid 5, and reflection occurs also in this part, and the reflected light also enters the 3 dB fiber coupler 3 from the end face B.

The 3dB fiber coupler 3 further combines the reference light incident from the end face A and the reflected light incident from the end face B, and combines the reference light incident from the end face A with the reflected light incident from the end face B.
Emits from.

The light emitted from the end surface E becomes a parallel beam by the collimating lens 8 and enters the photodetector 9.

The output of the photodetector 9 is stored in a waveform memory 10 and processed by an arithmetic unit 11.

The coherence length of the light emitted from the light source 1 is λ12/δλ= (1.3X10"-')2/(600
X10-8) (cm)=28 [μ mountain]. In other words, when the light emitted from the light source 1 is split into two and combined again, the optical path length difference between the two split lights is 28 μm.
Interference fringes occur only when: On the other hand, if the length β of the optical waveguide 4 to be measured is lcm, then 3d
The optical path length difference between the reflected light generated at the end surface B of the B-fiber coupler 3 and the input end C of the optical waveguide 4 to be measured and the reflected light generated at the output end D of the optical waveguide 4 to be measured is such that the refractive index n is 1.5. 2n1=3Ccm). Therefore, when the total reflection mirror 7 is moved away from the end face A in the beam direction, the end face B and the incident end C
The reflected light at and the reflected light at the output end D interfere with the reference light independently and sequentially.

Therefore, by adjusting the digit 1 of the total reflection mirror 7, the optical path length of the reference light and the optical path length of the reflected light from the output end D are almost matched, and then the total reflection mirror 7 is swept to several times the coherence distance. The interferogram is stored in the waveform memory 10. Thereby, the interferogram between the reference light and the light reflected by the output end D can be extracted while being distinguished from other points. The calculation device 11 calculates the inverse Fourier transform of this interferogram to obtain a reflectance spectrum.

Processing by the arithmetic device 11 will be explained.

The reflectance of the output end D at the optical frequency ν is R(ν), and the light .
If the spectrum of the light emitted from R1 is G(ν), and the delay time difference between the reference light and the light reflected by the output end D is τ, then the interferogram is: ■(τ)=S CG(ν>-.”w Possible xexp j(2πντ+φ(ν))]dν2c, c
, ・(1). Here, J = −1, φ(ν) is the phase spectrum C, C
．． is a conjugate complex number.

The reason R(ν) is included in equation (1) is that the interferogram is expressed as a correlation between the electric field amplitudes of the reference light and the reflected light. From equation (1), if we take the inverse Fourier transform H(ν) of ν) can be determined in advance, and if the wavelength dependence of the reflectance is clear, it can be determined by Fourier transformation of the reflected light from the end face B of the 3 dB fiber coupler 3. Therefore, the reflection spectrum R (ν) is found by

FIG. 2 shows the configuration of a light guide end face reflectance measuring device according to a second embodiment of the present invention.

The phase difference ψ of the light propagating within the two fiber tails on the output side of the 3 dB fiber coupler 3 changes randomly and rapidly due to temperature fluctuations and various disturbances. Therefore, even if the total reflection mirror 7 is moved at a constant speed and the interferogram is measured at regular time intervals, the optical path length difference changes randomly for each measurement.

Therefore, if the interference pattern is subjected to inverse Fourier transform as it is, distortion may occur in the reflectance spectrum.

Therefore, in this embodiment, a monitor means for measuring the relative optical path length difference between the reference light and the reflected light is provided. This monitoring means includes a He-Ne laser 12 as a reference light source whose emission spectrum is sufficiently different from that of the light source 1 and whose emission spectrum width is sufficiently narrow, and the emitted light of the He-Ne laser 12 is used as a reference light source. A gichroic mirror 13 is provided as means for combining the emitted light, and an AC oscillator 25, a voltage amplifier 26, and a PZT phase modulator 27 are provided as phase modulation means for applying phase modulation of frequency f to the reference light or reflected light. , a light isocum mirror 14, a photodetector 21, and a rotor are used as a means for receiving the combined light obtained by the 3 dB fiber coupler 3 and measuring the phase relationship between the frequency f component and the frequency 21 component included in the received light output. It is equipped with quin amplifiers 20 and 23.

At this time, a frequency component is included in the interference pattern due to phase modulation. In order to eliminate this, a lock-in amplifier 20 is provided as means for phase-detecting the interference signal obtained by receiving the combined light of the reference light and the reflected light.

Furthermore, a trigger circuit 24 is provided in order to adjust the measurement timing based on the output of the monitor means.

The PZT phase modulator 27 has a structure in which one arm of the 3 dB fiber coupler 3 is wound around a cylindrical electrostrictive vibrator.

The alternating current oscillator 25 generates an alternating current with a period f =10 kHz. The voltage amplifier 26 converts this AC signal into a peak-to-peak voltage 5
The voltage is amplified to 0 volts, and the electrostrictive vibrator of the PZT phase modulator 27 is expanded and contracted in the radial direction. Thereby, phase modulation is applied to the light propagating through this arm.

The light emitted from the He-Ne laser 12 is multiplexed with the light emitted from the light source 1 by a guichroic mirror 13 .

One of the lights divided into two by the 3 dB fiber coupler 3 is reflected by the total reflection mirror 7, and the other is reflected by the output end D of the optical waveguide 4 to be measured.

Even if a reflective coating is provided at the output end D, the coating is
．． In the case of 3 μm, the oscillation wavelength of the tle-Ne laser 12 is 0. Generally, the reflectance for a wavelength of 6328 μm is a sufficiently large value. Furthermore, even if the reflectance for this wavelength is small, the refractive index compensation liquid 5
It is possible to utilize the reflected light at the end face B of the 3 dB fiber coupler 3 or the human emission end C of the optical waveguide 4 to be measured without using the 3 dB fiber coupler 3.

The reflected light from the total reflection mirror 7 and the reflected light from the output end D are combined by the IB fiber coupler 3, and among this combined light, the wavelength 0. The component of 6328μ... is Guicroic Mira-14
and enters the photodetector 21.

Wavelength 0.0 by PZT phase modulator 27. The modulation degree of light of 6328 μm is φ. Then, the AC component J (t) included in the output of the photodetector 21 is J (t) = I. cos(φ.cos2πft+ψ
) −(4). Here, ψ is the phase difference between the two reflected lights, and due to the optical path length difference 2 of these reflected lights, ψ=2πZ/λ2 (λ2=0.6328μ[I1
) is given by

Expanding equation (4) using the Betzel function, we find that the AC component J (
The frequency d and 2f components in t) are respectively Kr(t) = I o J + (φo) sinψC
OS2πft , K2r(t) = r o J 2
(φo) sinψCOS2π(2f)tノ. Therefore, by detecting the f component and the 2f component by the mouth-to-mouth amplifiers 22 and 23, and performing appropriate voltage amplification and inversion, the following outputs are obtained: S,=Sosin ψ, S2=Socos ψ. However, S. is a constant.

Therefore, similarly to the rotary encoder, a trigger pulse is generated from the trigger circuit every time the optical path length difference 2 changes (increases or decreases) in one direction by λ/4. The waveform memory 10 samples the interferogram in synchronization with this pulse.

This makes it possible to perform sampling at a constant interval of λ/4 that is independent of variations in optical path length difference due to temperature variations or disturbances.

Furthermore, due to the phase modulation by the PZT phase modulator 27, the interferogram obtained as the total reflection mirror 7 moves is as follows: 1(r)=. f'[c(ν)−,'TrTO Xexp j(2πγfuφ(τ)〒φoCOS2πf
t) ldν. That is, the interferogram itself is phase modulated at the frequency f. Therefore, the lock-in amplifier 20 performs level detection at the frequency of the output of the light receiving element 9 to restore the interferogram of equation (1).

Similarly to the first embodiment, the arithmetic unit 11 stores the waveform memo j month 0.
The inverse Fourier transform is applied to the interface digram stored in , and the reflectance spectrum is calculated.

FIG. 3 shows an example of interferogram measurement.

In the second measurement, the waveguide length lcm is used as the optical waveguide 4 to be measured.
, a refractive index n=1.5 was used.

The interferogram at z=Q is due to the reflected light mainly occurring at the end face B of the 3 dB fiber coupler 3 and the reflected light generated at the input end C of the optical waveguide 4 to be measured. Since the end surface B and the incident end C are close to each other by 1 μ0 or less, they are measured as one interferogram.

The interferogram at z=3 cm is due to reflected light from the output end D of the optical waveguide 4 to be measured. FIG. 4 shows a reflectance spectrum obtained by inverse Fourier transform of this interferogram. In this example, wavelength 1
．． It was measured that the reflectance reached a minimum value of IO-3 at 3 μm.

In this example, the reference light and the phase-modulated reflected light are combined to perform homodyne detection, so the minimum detection reflectance is 10-
A dynamic range of 8, that is, -80 dB was obtained, and a detection sensitivity of more than 5 orders of magnitude was obtained compared to the conventional device.

〔Effect of the invention〕

As described above, the optical waveguide end face reflectance measuring device of the present invention can measure the wavelength dependence of the optical waveguide end face reflectance in a dynamic range of about 33 dB.

Therefore, it is particularly effective when used to optimize antireflection coatings for optical waveguides.

ZT phase modulator, 50... optical power meter

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an optical waveguide end face reflectance measuring device according to a first embodiment of the present invention. FIG. 2 is a diagram showing the configuration of an optical waveguide end face reflectance measuring device according to a second embodiment of the present invention. FIG. 3 is a diagram showing an example of interferogram measurement. FIG. 4 is a diagram showing a reflectance spectrum obtained from this interferogram. FIG. 5 is a diagram showing the groove configuration of a conventional light guide path end face reflectance measuring device. DESCRIPTION OF SYMBOLS 1... Light source, 2... Condensing lens, 3... 3dB fiber coupler, 4... Optical waveguide to be measured, 5... Refractive index compensation liquid, 6... Collimating lens, 7... ... Total reflection mirror, 9, 21... Photodetector, 10... Waveform memory, 11... Arithmetic device, 12... He-Ne laser, 13, 14-... Guicroic mirror, 20, 22,
23...Lock-in amplifier, 24...Trigger circuit,
25... AC oscillator, 26... voltage amplifier, 27...
... P Patent applicant Nippon Telegraph and Telephone Corporation σ Agent Patent attorney Nao Ide Takashi shoulder Ichii Oh example 1 times interferon film beach 1 constant tari fan 3 times foot measurement gL shot kiss height 4 times tonore

Claims (1)

[Scope of Claims] 1. A light source, means for inputting light emitted from the light source into an optical waveguide to be measured, and means for measuring reflected light reflected by an end face of the optical waveguide to be measured and appearing at an input end. An optical waveguide end face reflectance measuring device comprising: a means for branching a reference light from the light emitted from the light source; a means for combining the reference light with the reflected light; and a range in which the reference light interferes with the reflected light. and means for changing the relative optical path length of the reference light and the reflected light, and the measuring means includes means for inverse Fourier transforming an interference pattern caused by the change in the relative optical path length. An optical waveguide end face reflectance measurement device characterized by: