JPS63261120A - Optical pulse generating circuit and spectrum characteristics measuring instrument - Google Patents

Abstract

PURPOSE:To shorten a measurement time by cutting off an optical path of a wavelength component converged on a single focus by a waveguide. CONSTITUTION:A semiconductor laser 12 is driven by a driving circuit 11 including a modulating signal source and its light pulse output is made incident as a parallel beam on the optical circuit consisting of diffraction gratings 1 and 2, lenses 3 and 4, and a spatial distribution filter 5. The projection parallel beam light from this optical circuit is passed through an optical fiber 17 for optical compression and outputted as an output optical pulse. Consequently, the optical pulse outputted having nearly Lorentz waveform is diffracted by the diffraction grating and converged by the lens 3 and its focal plane goes to a Ferry plane of on optical frequency. This the filter 5 arranged in an optical frequency plane based upon said plane as position coordinates imposes modulation in a specific wavelength range. This modulated optical pulse is passed through the lens 4 and grating 2 to go to a parallel beam, which is passed through the fiber 17 for optical pulse compression and guided out as an output optical pulse having desired waveform.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to an optical pulse generation circuit that generates ultrashort optical pulses of arbitrary waveforms using a pair of angular dispersion elements, and a semiconductor that generates the ultrashort optical pulses. The present invention relates to a laser spectral characteristic measuring device.

[Conventional technology]

Ultrashort optical pulses are used not only for large-capacity, ultrahigh-speed optical pulse transmission, but also as sampling light sources in optical sampling, which is one of the methods for observing high-speed optical phenomena.

In such application fields, it is necessary to generate a waveform with a desired shape, and in long-distance, high-capacity optical fiber transmission, it is required to further improve the mode stability of the semiconductor laser that serves as the optical pulse light source. ing.

The spectrum of a single-mode semiconductor laser used as an optical pulse light source in a long-distance, large-capacity optical fiber transmission system is not a single mode in the strict sense. Most lasers have a single mode that satisfies the system requirements, but in some lasers, a sub-mode with a wavelength different from the main mode may emit strong light under certain temperature and bias conditions.

According to the inventors' observations, this secondary mode oscillation phenomenon occurs with a probability of about 10-6 even if it occurs frequently, so the average power is very weak and cannot be detected by direct current spectrum measurement. was difficult.

However, in terms of the method, it is not possible to allow sub-mode light emission which occurs with a probability of 10-12 or more.

Therefore, observation of this sub-mode emission phenomenon requires dynamic spectrum measurement, and conventionally, dynamic spectrum measurement of semiconductor lasers has been carried out using a measuring device shown in FIG.

The semiconductor laser spectrum measuring device shown in FIG. 2 extracts output light from a semiconductor laser 12 modulated and oscillated by a drive circuit 11 including a signal source through an optical fiber 17 and supplies it to a spectrometer 13. The detected spectral components are detected by a photodetector 14 and counted by a counter 15. This semiconductor laser 12 is placed in a constant temperature bath 16, and the oscillation spectrum components of the semiconductor laser can be measured over a temperature range that can be measured by the method. The counter 15 is configured to be synchronized with the signal source of the drive circuit 11 and triggered by an input pulse, and in order to avoid unnecessary triggering due to noise, the trigger level needs to be about half of the main mode pulse. Note that this counter 15 is capable of measuring pulses having a band approximately equal to the clock frequency of the transmission bit rate in the system.

[Problem that the invention seeks to solve]

However, in this spectrum measurement device, the spectrometer 1
It is necessary to set the wavelength resolution of 3 to several angstroms, scan the measurement wavelength, and confirm that pulses are not counted in wavelength regions other than the main mode. In this case, it is necessary to change the light source temperature over the temperature range that can be measured by the method, and the conventional method for measuring the spectrum of a semiconductor laser has the disadvantage that the measurement takes an enormous amount of time.

In addition, a spectrometer can extract a desired wavelength component, but other components are focused at different positions depending on the wavelength even if they are focused with an element like a normal lens. It was not possible to refocus the wavelength components into a single focus.

One possible way to solve this problem of not focusing light on a single focal point is to use a large number of photodetectors arranged in a row to detect light with wavelength components that are focused at different positions. arranging them densely and processing the output signals of a huge number of channels would complicate the equipment and its feasibility is questionable.

As a method that does not use a spectrometer, it is possible to remove a specific wavelength component using a Fabry-Perot etalon with a thickness of several tens of capes, but in order to change this wavelength, the thickness of the etalon must be at least It is necessary to change the wavelength by more than half a wavelength, and there is currently no etalon that can do this.

In addition, the waveform of the optical pulse from the semiconductor laser for generating ultrafast optical pulses is determined by the physical properties of the semiconductor laser, and the optical pulse has a roughly Lorentzian waveform, so it is necessary for measurements, sampling, etc. It was necessary to perform waveform shaping or compression to output the optical pulses with the desired waveforms.

As a method for extracting the spectral components of a parallel beam of light, the incident light diffracted by a diffraction grating is focused on a single focal point by a lens, the temporal Fourier component is spatially dispersed, and this dispersion is A technique has been proposed in which the wavelength components are adjusted and synthesized using a filter, etc., and the waveform of the optical pulse is shaped.

Literature, "Shaping of optical pulses using a spectrometer" Nobuyuki Kagi, Hiroshi Ema, Fujio Shimizu, Shun Tanaka - March 30, 1986
The present invention utilizes this optical pulse shaping technology using a diffraction grating and lens to generate optical pulses of arbitrary waveforms in an ultrahigh-speed optical pulse generation circuit. It is an object of the present invention to provide an optical pulse generation circuit that can be obtained and a semiconductor laser spectral characteristic measuring device that shortens measurement time without requiring wavelength region scanning.

[Failure to solve the problem]

The first invention includes an optical pulse generator that generates parallel beam light, a pair of angular dispersion elements into which the parallel beam light is incident and the parallel beam light is emitted, and optical pulse compression of the emitted parallel beam light. A filter is arranged in the optical path of the pair of angular dispersion elements to block the optical path of a specific spectrum, and a lens for forming a focal point is installed on the filter. .

The second invention includes an optical pulse generating section that generates a parallel beam of light, a pair of angular dispersion elements into which the parallel beam of light is incident and the parallel beam of light is emitted, and a light detection means that detects the emitted light, 1) A filter for blocking the optical path of a specific spectrum is disposed in the optical path of the pair of angularly dispersive elements, and a lens for forming a focal point is provided on the filter.

In this second invention, it is preferable that the filter is a light absorber or a photodetector.

[Effect]

A light beam that has been wavelength-dispersed by a diffraction grating is shifted in parallel depending on the wavelength, but if it is focused using a lens that is larger than the beam diameter, it can be focused at a single focal point.

The present invention makes use of this phenomenon and can select a specific wavelength region of a light beam by blocking the optical path of wavelength components focused on a single focal point using a filter.

By blocking the optical path in this specific wavelength range, it is possible to shape the optical pulse of approximately Lorentzian waveform generated by the semiconductor laser into an optical pulse of arbitrary waveform and output it.

In addition, by absorbing light in a specific wavelength range with a light absorber, the wavelength component to be measured can be extracted, and by detecting light in a specific wavelength range with a photodetector, the dynamic spectrum of the semiconductor laser can be determined. It makes it possible to measure characteristics.

〔Example〕

The present invention will be explained in detail below.

FIG. 3 explains the principle of an optical circuit for shaping the waveform of optical pulses using a lens and a diffraction grating pair, which is used in the present invention.

This waveform shaping optical circuit has a focal length (f) of one quarter of the distance between diffraction gratings 1 and 2.
A spatial distribution filter 5 that modulates the amplitude and phase of light in the focal plane between the lenses 3 and 4 by disposing a number of convex lenses 3.4 apart from the diffraction grating 1.2 by the respective focal lengths.
is arranged.

At this time, the plane perpendicular to the optical axis at the center of the pair of diffraction gratings becomes a so-called Fourier plane. That is, position coordinates on the Fourier plane correspond to spatial frequency coordinates, but in this case, there is a feature that wavelength coordinates correspond to position coordinates on the Fourier plane.

Since a spatial distribution filter 5 that modulates the amplitude and phase of light is arranged on this Fourier plane, the input optical pulse is modulated on the wavelength axis by passing through this spatial distribution filter 5, and then the second The wavelengths are synthesized by the diffraction grating 2.

As a result, the output optical pulse is output as an inverse Fourier transform waveform of the spatial distribution of transmitted light intensity and phase on the spatial distribution filter surface, making it possible to shape the optical pulse waveform and perform waveform compression. .

Although each lens is shown as one, it can be realized by configuring it as a lens group to eliminate chromatic aberration. Also, since this optical circuit is used for light beams in a narrow frequency band, chromatic aberration can be eliminated. The problem has little impact, and there is no problem with practicality. Further, as the spatially distributed filter, a spatially distributed amplitude filter and a spatially distributed phase filter can be used simultaneously or only one of them can be used.

The present invention that specifically utilizes this principle will be described in detail below.

(Embodiment 1) FIG. 1 shows the configuration of an arbitrary waveform optical pulse generation circuit of the present invention.

The semiconductor laser 12 is driven by a drive circuit 11 including a modulation signal source, and its optical pulse output is input as a parallel beam into an optical circuit consisting of a diffraction grating I, 2, a lens 3.4, and a spatial distribution filter 5. Ru. The parallel beam light emitted from this optical circuit passes through an optical fiber 17 for optical pulse compression and is output as an output optical pulse.

The semiconductor laser 12 is driven by the drive circuit 11 using a gain switching method and outputs a light pulse. The optical pulse output with this approximately Lorentzian waveform is diffracted by the diffraction grating 1 and condensed by the lens 3, and its focal plane becomes the Fourier plane of the optical frequency.

The spatial distribution filter 5, which consists of a spatially distributed amplitude filter that adjusts the amplitude of light and a spatially distributed phase filter that adjusts the phase of the light, arranged on the optical frequency plane with this Fourier plane as the position coordinate, is used to detect a specific wavelength range. Modulation is performed. The modulated light pulse passes through the lens 4 and the diffraction grating 2, becomes a parallel light beam, passes through an optical fiber 17 with normal dispersion that compresses the light pulse, and is extracted as an output light pulse with a desired waveform. It will be done.

This optical fiber 17 compensates for wavelength chirping toward the longer wavelength side accompanying the optical pulse generated by the semiconductor laser 12, and compresses the pulse waveform. In addition,
The center wavelength of a single optical pulse is shifted, and the phenomenon in which the oscillation wavelength changes over time is called chirping.

In principle, the waveform compression function of the optical fiber 17 can be performed by the spatially distributed phase filter of the spatially distributed filter 5 described above, but in reality, a large phase shift is required, and in reality, Since such a spatial distribution filter cannot be realized, an optical fiber 17 having normal dispersion for waveform compression is coupled to the outside of the optical circuit that performs waveform shaping.

With this configuration, the optical pulses oscillated by the semiconductor laser 12 can be output as optical pulses with arbitrary waveforms.

(Embodiment 2) FIG. 4 shows the above-mentioned optical circuit used as the spectrometer 13 in the semiconductor laser spectral characteristic measuring device shown in FIG. A light absorber 8 having a size a is arranged.

At this time, when the groove spacing of the diffraction grating is d and the diffraction order is m, the light absorber 8 absorbs light with a wavelength width given by the formula (ad)/(2mf).

Assuming that the number of grooves in the diffraction grating is 600/ml and the focal length is 5 cm as general values, a 30 trm light absorber absorbs light with a width of 1 nm. That is, the light emitted from the diffraction grating pair is light from which this lnm spectrum is removed.

However, as shown in FIG. 4, an approximation is made here in which the angle at which the diffracted light is diffracted is approximately perpendicular. The wavelength to be absorbed can be selected by adjusting the angle of incidence on the diffraction grating.

The optical system of the optical circuit shown in FIG. 4 is symmetrical with respect to the central plane of the pair of diffraction gratings, so the central plane is
If the structure is such that the light absorber 8 is arranged within that plane,
The optical system can be further simplified as shown in FIG.

The spectrum of the main mode of a pulse-modulated single-mode semiconductor laser used as a transmission light source in a long-distance, high-capacity optical fiber transmission system is approximately several angstroms long, including its tail. Since the components can be extracted as a parallel beam of light, the light can be focused on a single focal point using a lens and the spectrum of the secondary mode of the semiconductor laser can be measured.

An optical circuit using the above-mentioned light absorber 8 is shown in FIG.
By applying this instead of , the spectroscope 13 becomes unnecessary, scanning in the wavelength region becomes unnecessary, and the time required to measure the dynamic spectrum characteristics of a semiconductor laser can be significantly reduced.

In this embodiment, a concave mirror may be used instead of the convex lens 3.4, and furthermore, the diffraction grating 1.2 itself may have a concave shape having a light focusing function. It is sufficient that the diffraction grating and lens optically perform a Fourier transform function.

(Example 3) In Example 3 of the present invention, a photodetector was used in place of the light absorber of Example 2.

In this case, it is convenient to use a configuration in which a light absorber 8 is arranged on a plane mirror 6 and the optical path is folded back as shown in FIG. Of course, a photodetector may be substituted for the light absorber 8 disposed in the middle of the diffraction grating shown in FIG. fabricating such a photodetector is technically difficult and uneconomical.

Further, as shown in FIG. 6, lens 3. A configuration may also be adopted in which a small mirror 9 is placed on the Fourier plane between and 4, and the wavelength portion reflected by this mirror is detected by the photodetector 10.

FIG. 7 shows the configuration of a specific semiconductor laser spectrum measuring device using a folded configuration and arranging a photodetector.

A slit is provided in the plane mirror 6, and a photodetector 20 is placed therein. Further, the output light beam output from the half mirror 7 is detected by a photodetector 14. The input beam light outputted from the semiconductor laser 12 driven by the signal outputted from the drive circuit 11 is outputted from the half mirror 7 through an optical circuit composed of a diffraction grating 1, a lens 3, and a plane mirror 6.
It is detected by the photodetector 14.

Similarly, a photodetector 20 provided in the slit of the plane mirror 6 detects light of a specific frequency. The output of the photodetector 20 is counted by a counter 21, and the output of the photodetector 14 is counted by a counter 15. The output of the counter 21 and the output of the signal source of the drive circuit 11 are exclusive ORed by an exclusive OR circuit 22, and the output is counted by a counter 23.

Counter 15 and counter 23 are adjusted by a threshold controller 24.

Now, only the main mode power is detected by the photodetector 20, and the remaining spectrum power is detected by the photodetector 14. The output pulses from the photodetector 20 are counted by a counter 21, and the output of the counter 21 and the signal from the signal source of the drive circuit 11 are exclusive ORed, and the threshold controller 24 performs the exclusive ORing. The threshold value of the counter 21 is adjusted so that the output of the circuit 22 becomes zero or minimum. If the output pulses of this exclusive OR circuit 22 are counted by another counter 23, it is possible to count errors caused by missing pulses in the main mode.

On the other hand, if the threshold value of the counter 15 that counts mode output pulses of wavelengths other than the main mode is made the same as the threshold value of the counter 21, it is possible to automatically optimize the threshold value.

This makes it possible to simultaneously count pulse errors caused by both pulse omission in the main mode and light emission in the sub mode.

Although it is desirable that the two photodetectors 14 and 20 have the same characteristics, if their characteristics differ, a compensation amplifier may be placed at the subsequent stage.

A spectral measurement device using the photodetector 14.20 described above can be used as part of a wavelength stabilization circuit for a single mode light source. That is, as shown in FIG. 8, a small plane mirror 30 is adjusted in advance so as to reflect the main mode, and two photodetectors 31 and 32 are placed on both sides of this plane mirror 30. The outputs of the photodetectors 31 and 32 are input to a feedback circuit 34, and the output of the feedback circuit 34 controls a single mode light source 35.

The output beam of the optical circuit is input into optical fiber 36.

With this configuration, the main mode light is not detected by the photodetectors 31 and 32 when it is stable. However, if the oscillation wavelength of the main mode changes to the long wavelength side due to fluctuations in ambient temperature, the photodetector 31 will be activated, and if the oscillation wavelength of the main mode changes to the short wavelength side, the photodetector 31 will be activated.
2 detects the main mode power, the oscillation frequency of the single mode light source 35 can be stabilized by applying feedback in the direction of decreasing these signals in the feedback circuit 34.

This single-mode light source stabilization circuit can be used as a light source stabilization circuit for suppressing an increase in crosstalk due to wavelength fluctuations, for example, in wavelength multiplexing transmission.

〔Effect of the invention〕

As described above, the present invention can generate ultrashort optical pulses with arbitrary waveforms, and can significantly save labor and perform dynamic spectrum measurements of semiconductor lasers in a short time.

By using the present invention, it is possible to stabilize the mode of a transmitting light source in a large-capacity optical transmission system, and it is also possible to obtain optical pulses of arbitrary waveforms for measurement.

Furthermore, by inserting the dynamic spectrum measurement system using the optical circuit of the present invention into a wavelength-multiplexed optical local area network, it has become possible to apply it to nodes that transmit and receive only specific wavelength components.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing Embodiment 1 of the present invention. FIG. 2 is a configuration diagram of a conventional semiconductor laser spectrum measuring device. FIG. 3 is a diagram showing the principle of waveform shaping of the present invention. FIG. 4 is a diagram showing the configuration of an optical circuit according to Embodiment 2 of the present invention. FIG. 5 is a configuration diagram showing a modification of the optical circuit according to the second embodiment of the present invention. FIG. 6 is a configuration diagram showing an optical circuit according to Embodiment 3 of the present invention. FIG. 7 is a configuration diagram of a device for measuring spectral characteristics of a semiconductor laser using Example 3 of the present invention. FIG. 8 is a configuration diagram of a wavelength stabilizing circuit for a semiconductor laser using Embodiment 3 of the present invention. l, 2... Diffraction grating, 3.4... Lens, 5...
Spatial distribution filter, 6... Plane mirror, 7... Half mirror 18... Light absorber, 9... Reflector, 10.1
4.20.31.32... Photodetector, 11... Drive circuit, 12... Semiconductor laser, 13... Spectrometer, 1
5.21.23...Counter, 17.36...Optical fiber, 22...Exclusive OR circuit, 3o...Plane mirror, 34...Feedback circuit, 35...Single mode light source . 2 υ Example light pulse leftover mouth trace broom 1 M] ζ Mika 1 Kui row L [Bow 1j Igoto explanatory diagram No. 3 Mouth large abolition example 2 Light turning tower tail 4 Diagram large example 2 Preliminary circuit section 5 Large box example 39th Yukiho 6 Written amendment of figure procedure June 25, 1988 1. Indication of the case Patent Application No. 94539 of 1988 2. Name of the invention Optical pulse generation circuit and spectral characteristic measuring device 3. Person making the amendment Relationship to the incident Patent applicant address 1-1-6 Uchisaiwai-cho, Chiyoda-ku, Tokyo Name (422) Nippon Telegraph and Telephone Corporation Representative Makoto Fuji
Kou 4, Agent address: 26-18 Sekimachi Kita 2-chome, Nerima-ku, Tokyo.
-m-2,. Name Patent Attorney (7823) Naotaka Ide 2', F8, Contents of amendment The drawings will be amended as shown in the attached drawings. 9. Attached documents drawings (Figures 1, 3, and 6) 1 copy 4 rows of lights 7 times Shoulder silkworm explanatory diagram Shoulder 3 l Box 491j 3 Light turning tower 'M 6 Figure

Claims (4)

[Claims] (1) An optical pulse generator that generates a parallel beam; a pair of angular dispersion elements into which the parallel beam is incident; and an optical fiber that compresses the optical pulse of the output parallel beam. An optical pulse generation circuit comprising: a filter for blocking an optical path of a specific spectrum in the optical path of the pair of angular dispersion elements; and a lens for forming a focal point on the filter. (2) A light pulse generating section that generates a parallel beam of light, a pair of angular dispersion elements into which the parallel beam of light is incident and the parallel beam of light is emitted, and a light detection means that detects the emitted light, the angular dispersion described above. 1. A spectrum measuring device, characterized in that a filter for blocking an optical path of a specific spectrum is disposed in an optical path of a pair of elements, and a lens for forming a focal point is provided on the filter. (3) Claim No. (2) in which the filter is a light absorber
Spectral measuring device described in Section 1. (4) Claim No. (2) in which the filter is a photodetector
Spectral measuring device described in Section 1.