Optical wavelength converter

Abstract: A wavelength converter having the light source formed of the semiconductor laser (1) and the nonlinear optical element (2), wherein, the oscillation light therefrom is incident to the nonlinear optical element (2) as an input fundamental wave, and the oscillation wavelength is changed by the very small variation current added to the injection current to the semiconductor laser so that the input fundamental wavelength to the nonlinear optical element (2) is changed and that the oscillation wavelength of the semiconductor laser is pulled in the input fundamental wavelength at which the wavelength conversion efficiency is the maximum.

Description: Field of the Invention This invention relates to an optical wavelength converter in accordance with the precharacterizing parts of independent claims 1 and 6. Such a wavelength converter is known from US-A-5 111 468. Background of the Invention When a non-linear optical element has a narrow light harmonic generation wavelength band, a semiconductor laser is sometimes used as the fundamental wave light source. When a semiconductor is used as the fundamental wave light source, the oscillation wavelength drifts with the variation of the ambient temperature. This makes it difficult to generate a second harmonic at the highest efficiency at all times. To alleviate this problem, the semiconductor laser used as the fundamental wave light source and the non-linear optical element have so far been maintained at a constant temperature. However, even when these components are maintained at constant temperature, wavelength variation cannot be fully reduced. Thus, variations are introduced into the wavelength conversion efficiency and the light harmonic output power making it difficult to maintain these factors at the maximum. US-A-5 111 468 discloses a wavelength converter which uses a phase-matched second harmonic generation (SHG) process to generate a second laser beam by frequency doubling the central carrier frequency of the diode laser source and a portion of the laser beam incident to a non-linear optical element. Said portion of the laser beam is directed to a detector which generates a RF-signal arising from the difference in phase shifts or amplitudes between the RF-frequency side bands caused by any mismatch between the central carrier frequency of the semi-conductor laser and the resonance frequency of the resonator, i.e. the non-linear optical element. A RF-mixer receives the amplified RF-signal from the detector and mixes it with a signal from a low amplitude RF-source (local oscillator) for a demodulation and generation of an error signal. The latter is used as a feedback signal for tuning the laser injection current or the diode temperature so that a central carrier frequency of the diode laser is automatically adjusted for tracking any changes in the longitudinal resonance frequency. Japanese J. Applied Physics, volume 31, no. 2B, February 1992, pages 513-570 discloses a FM sideband locking method to achieve frequency locking of a single-mode green laser. An electro-optic phase modulator is used generating FM sidebands at the frequency of 10 MHz. Reflected light, obtained from a TGG Faraday rotator and a polarization beam splitter is detected by a photodetector. Then the detected signal is locked-in for generating a frequency error signal. The latter is fed into a PZT mirror positioner to adjust the resonance frequency of a BBO carrier (BBO = Beta Barium Borate). The specific feature of these known wavelength converters is different from the present application in that the known wavelength converters are rendering the resonance wavelength in the cavity coincident with the fundamental wavelength. Summary of the Invention An object of this invention is to provide a wavelength converter having improved stability. This object is, according to a first aspect of the present invention, achieved by the features specified in independent claim 1. Depending claims 2 to 5 are specifying advantageous developments thereof. Further the above object is, according to a second aspect of the present invention, achieved by a wavelength converter specified in independent claim 6. Claims 7 to 10 depending thereof are each specifying advantageous developments thereof. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawing. Brief Description of the Drawings FIGURE 1 is a block diagram of one embodiment of the wavelength converter of the present invention. FIGURE 2 is a block diagram of another embodiment of the wavelength converter of the present invention. FIGURE 3 is a graph showing the relation of power derived from the nonlinear optical element to the wavelength of the fundamental wave light. FIGURE 4 is a graph showing the relation between the second harmonic output power from the nonlinear optical element and the wavelength. FIGURE 5 is a diagram explaining the very small variation of the fundamental wave length to the nonlinear optical element and the output variation thereby. FIGURE 6 is a diagram explaining the very small variation of the fundamental wavelength to the nonlinear optical element and the variation of the second harmonic output thereby. FIGURE 7 is a diagram showing the relation between the fundamental wavelength and the inner product of the fundamental wave very small variation signal Δλω and the fundamental wave output power signal ΔP outω . FIGURE 8 is a diagram showing the relation between the fundamental wavelength and the inner product of the fundamental wave very small variation signal Δλω and the light high frequency output power signal ΔP out2ω . FIGURE 9 is a block diagram of another embodiment of the waveform converter of the invention. FIGURE 10 is a block diagram of another embodiment of the waveform converter of the invention. FIGURE 11 is a block diagram of another embodiment of the waveform converter of the invention. FIGURE 12 is a perspective view of one example of the nonlinear optical element. FIGURE 13 is a perspective view of one example of the external frequency modulator. Description of the Invention While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. Where like reference numerals are used in the several views of the drawing, these numerals represent identical or similar components. In those cases, the various components are only described in connection with one figure of the drawing. For convenience of explanation, before detailed embodiments of the invention are described, the basic principle for making the oscillation wavelength of the semiconductor laser coincident with the wavelength at which the conversion efficiency of the nonlinear optical element is the maximum will be discussed using FIGURES 3, 4, 5, 6, 7 and 8. Now let it be considered that a fundamental wave light beam of input power P in ω and wavelength λω is fed to the nonlinear optical element SHG (Second Harmonic Generator) for generating the second harmonic. FIGURES 3 and 4 respectively show the fundamental wave output power P out ω and second harmonic output power P out 2ω from the SHG with respect to the fundamental wave input wavelength λω. In this case, the SHG is of the quasi-phase matching structure. From these figures, it will be seen that the wavelength-dependency of each of the fundamental wave output power and harmonic wave output power has a peak at a certain wavelength (λp) at which the conversion efficiency is the maximum. Thus, when the fundamental light wave incident to the SHG has a very small variation signal (Δλω), the light wave output power signal (ΔP out ω) and light second harmonic wave output power signal (ΔP out 2ω) from the SHG are as shown in FIGURES 5 and 6, respectively. The inner products taken between Δλω and the ΔP out ω and ΔP out 2ω are dependent on wavelength as shown in Figs.7 and 8. The curves shown in these figures are first-order differential of the characteristic curves shown in FIGURES 3 and 4. These differential curves have positive and negative wavelength regions (regions A in these graphs) around the wavelength λP at which the conversion efficiency is the maximum. When the fundamental light wave input center wavelength λω is present within this region A, the information of whether the wavelength λω is higher or lower than the wavelength λP can be obtained as the positive or negative from the signals shown in FIGURES 7 and 8. Thus, if the input light wavelength λω is changed in the direction for the coincidence with λP by negative feedback, the λω and λP can be always coincident with each other. The basic arrangement of one embodiment of the invention is shown in FIGURE 1. In FIGURE 1, the light source 1 is a semiconductor laser (LD) 1. The semiconductor laser 1 is supplied with and modulated by a very small variation current signal Δλω of a frequency f ref , for example, several tens of KHz in the range from 100 Hz to 100 MHz from a local oscillator 4. This very small variation current signal Δλω has an AC component of, for example, 0.1% in the range from about 0.001% to 10% with respect to the DC component of the injection current to be supplied to the semiconductor laser 1. In this case, since the output wavelength from the semiconductor laser LD is changed substantially linear with the change of the injection current, the output wavelength from the semiconductor laser LD is modulated directly by the very small variation current. The modulated output light ω from the semiconductor laser 1 is incident to the nonlinear optical element 2, by which the second harmonic 2ω, for example, is produced by wavelength conversion. This nonlinear optical element 2 also produces part of the incident light ω without wavelength conversion. A mirror 5 is disposed after the nonlinear optical element 2. In this embodiment, this mirror 5 is a dichroic mirror which allows the second harmonic light of wavelength 2ω to be passed therethrough but reflects the incident light ω of a longer wavelength to an optoelectric transducer 6. The light reflected from the mirror 5 is introduced to an optoelectric transducer 6 such as a photodiode, thereby converting the light into an electric signal. This electric signal is used as the variation current signal ΔP out ω which changes with the change of the fundamental wavelength from the semiconductor laser 1 to the nonlinear optical element 2. The characteristics of nonlinear optical element 2 are shown in FIGURE 5. An error signal is obtained by the correlation between the current variation signal ΔP out ω from the optoelectric transducer 6 and the very small variation current Δλω from the local oscillator 4. In this embodiment, the ΔP out ω and Δλω are supplied to a multiplying circuit 7 which then produces the inner product of both signals ΔP out ω and Δλω as illustrated in FIGURE 7. The output signal from the multiplying circuit 7 is supplied to a control circuit 8. Thus, the injection current is negatively fed back to the light source 1 (the semiconductor laser LD) so that the output wavelength, or the light input wavelength Δλω to the nonlinear optical element 2 can be made coincident with the λP. While in the embodiment of FIGURE 1 the semiconductor laser LD of the light source 1 is directly modulated by the very small variation current, the light from the light source 1, may alternatively be supplied to an external frequency modulator 3 as shown in the basic arrangement of FIGURE 2. In this embodiment, the frequency-modulated light is incident to the nonlinear optical element 2 as an input fundamental wave. The very small variation current from the local oscillator 4 is supplied to the external frequency modulator 3 as a frequency modulation signal, thus frequency-modulating the incident light to the nonlinear optical element 2. In the arrangement of FIGURE 2, the same error signal detection and negative feedback as in FIGURE 1 are performed. In FIGURE 2, like elements corresponding to those in FIGURE 1 are identified by the same reference numerals and will not be described. While in the embodiments of FIGURES 1 and 2 the fundamental wavelength output from the nonlinear optical element 2 is detected to produce the error signal, the waveform-converted output light from the nonlinear optical element 2, or the light passed through the mirror 5 may be detected to produce the error signal as shown in FIGURES 9 and 10. In FIGURES 9 and 10 the second harmonic signal 2ω passes through mirror 5 to the optoelectric transducer 6. Otherwise, in FIGURES 9 and 10, like elements corresponding to those in FIGURES 1 and 2 are identified by the same reference numerals and will not be described. FIGURE 11 is a block diagram of a specific circuit arrangement of the wavelength converter of the invention shown in FIGURE 1 or 9. In FIGURE 11, element 10 represents a driver circuit for driving the semiconductor laser LD of the light source 1. After the semiconductor laser LD, an isolator 11 is provided to prevent the return of light to the laser LD. The LD driver 10 adds a very small variation current to the injection current driving the laser LD. This very small variation current is provided by the signal of, for example, 40 kHz from the local oscillator 4, changing the oscillation wavelength from the semiconductor laser LD by a very small amount. The oscillation light from the semiconductor laser LD is incident to the nonlinear optical element 2 of, for example, QPM (quasi-phase matching) type, as a fundamental wave. The nonlinear optical element 2 produces the second harmonic. This second harmonic light is detected by the optoelectric transducer 6 such as a photodiode. The detected signal is amplified by an amplifier 12 and fed to a phase shifter 13, where it is synchronized with the signal from the local oscillator 4. The output from the phase shifter and the signal from the local oscillator 4 are supplied to the multiplying circuit 7, and the output therefrom is fed to a low-pass filter 14 of a cutoff frequency fc of, for example, 1 kHz. The output signal from the low-pass filter is fed to an integrator 15 of a cutoff frequency fc of, for example, 1 kHz. The output signal from the integrator is added to the oscillation output from the local oscillator 4, and fed to the driver circuit 10, thus making the negative feedback to the laser LD as previously described with reference to the basic arrangement of FIGURE 1. FIGURE 12 shows one example of the nonlinear optical element 2 of quasi phase matching (QPM) type, for example, SHG. An optical waveguide 21 is formed in a nonlinear optical crystal of KTP (Potassium Titanyl Phosphate - KTiOPO 4 ) or the like formed of a substrate 20 in which the Z-axis (C-axis) is taken in the width direction. In addition, polarity-inverted structural portions 22 are formed in the crystal to cross, for example, the optical waveguide 21 in its width direction with their polarities alternately inverted. The external frequency modulator 3 used in Figs.2 and 10 has an optical waveguide 31 provided in a nonlinear crystal substrate 30 as shown in FIGURE 13. The optical waveguide conducts the laser beam fed from the semiconductor laser LD. An electrode 32 is provided on, for example, both sides of the optical waveguide. A modulating voltage is applied to these electrodes. The electric field applied to the waveguide 31 controls the frequency of the guided light by use of its optoelectric effect, or by changing the refractive index to the guided light. This external frequency modulator 3 may be formed in the same crystal to be integral with the nonlinear optical element 2 of, for example, QPM type given in FIGURE 12. That is, the electrode 32 given in FIGURE 13 is provided on both sides of the optical waveguide 21 as indicated by the chain line in FIGURE 12, or the external frequency modulator 3 is built up within the nonlinear optical element 2 which is used as, for example, SHG. In addition, the nonlinear optical element 2 of the above construction can be formed as the SHG of Cherenkov radiation type, and the nonlinear optical crystal is not limited to KTP, but may be LiTa x Nb 1-x O 3 (0≤x≤1) . According to the arrangement of the invention, since the oscillation wavelength of the semiconductor laser LD is electrically controlled to coincide with the wavelength at which the conversion efficiency of the nonlinear optical element 2 is the maximum, it is not necessary to severely control the temperature of the semiconductor laser LD and nonlinear optical element 2. In addition, since the reference wavelength in the wavelength control is always the wavelength at which the conversion efficiency of the nonlinear optical element is the maximum, it is possible to suppress, for example, the drift of the intensity of the light harmonic output in the nonlinear optical element 2, and thus to realize a stable system at all times. Since this system itself can be formed of only a simple electric control circuit, it is easy to produce the system. Furthermore, in the nonlinear optical element 2 having the quasi phase matching structure QPM, the conversion efficiency can be increased by the increase of the polarity inversion region length, but there is a problem that the light harmonic generation wavelength band becomes narrow. Since this method can be applied to the nonlinear optical element of a very narrow wavelength band, it is possible to stabilize the nonlinear optical element with a larger conversion efficiency. Also, it it is possible to modulate by the very small signal frequency used in the wavelength control or by a frequency higher than the frequency band (f) in the electric control system. That is, it is possible to make the high-frequency intensity modulation directly by the injection current to the semiconductor laser (fm>f, where fm is the modulation frequency).

Claims: A wavelength converter comprising: a light source (1) formed of a semi-conductor laser (LD); a non-linear optical element (2), wherein oscillation light from said semi-conductor laser (1) is incident to said non-linear optical element (2) as an input fundamental wave and the oscillation wavelength of said semi-conductor laser (1) is changed by an injection current to said semi-conductor laser, said injection current being added with a variation current (Δλω) from a local oscillator (4) which changes the wavelength (λω) of said input fundamental wave (ω) to said non-linear optical element (2), characterized in that said local oscillator is such that said variation current (Δλω) added to said injection current is so small that the output wavelength (λω) from said semi-conductor laser (1) is changed substantial linearly with the change of the injection current due to said added variation current and a light signal corresponding to the added variation current (Δλω) is derived from an output of said non-linear optical element (2), opto-electric conversion means (6) is provided at the output side of said non-linear optical element (2) to convert the variation current derived from said light signal to a corresponding electric signal, correlation means (7) is provided to detect a correlation between said small variation current (Δλω) added to the injection current to said semi-conductor laser and said electric signal resulting from said opto-electric conversion of the derived variation current component of said fundamental wave from said non-linear optical element or of an output wave from said non-linear optical element which is wavelength converted, error signal generation means (7) are provided for obtaining an error signal from the difference between said input fundamental wavelength to said non-linear optical element (2) and the fundamental wavelength at which the wave length conversion efficiency of said non-linear optical element is the maximum and which can be obtained from said correlation, and negative feedback means (8) are provided to negatively feedback said error signal in the direction in which the wavelength (λω) of said input to said non-linear optical element coincides with the fundamental wavelength at which the wavelength conversion efficiency of said non-linear optical element is the maximum, whereby the oscillation wavelength of said semi-conductor laser is made equal to the wavelength at which the wavelength conversion efficiency of said non-linear optical element is the maximum. A wavelength converter according to claim 1, wherein said non-linear optical element is a light harmonic generating element. A wavelength converter according to claim 1, wherein said non-linear optical element has a quasi-phase matching structure. A wavelength converter according to claim 1, wherein said non-linear optical element is a Cherenkov-type non-linear optical element. A wavelength converter according to claim 1, wherein said non-linear optical element is made of LiTa x Nb 1-x O 3 . A wavelength converter comprising: a semiconductor laser light source (1), a non-linear optical element, an external frequency modulator (3), and an optical path, wherein light from said light source (1) is fed to said external frequency modulator (3), the output light from said external frequency modulator (3) is incident to said non-linear optical element (2) as an input fundamental wave, and a modulation signal is supplied to said external frequency modulator (3) which changes the wavelength of said input fundamental wave to said non-linear optical element, characterized in that said modulation signal is a very small variation frequency modulation signal (f ref ) from a local oscillator (4) of several tens of kHz, and a light signal corresponding to said very small variation modulation signal is derived from an output of said non-linear optical element, opto-electric conversion means (6) is provided to convert the derived light signal into a corresponding electric signal, correlation means (7) is provided to detect a correlation between said small frequency modulation signal supplied to said external frequency modulator and an electric variation signal derived from said electric output signal of said opto-electric conversion means, error signal generation means (7) is provided for obtaining an error signal from the difference between said input fundamental wave to said non-linear optical element (2) and the fundamental wavelength at which the wave length conversion efficiency of said non-linear optical element is the maximum and which can be obtained from said correlation, and negative feedback means (8) are provided for negatively feeding back said error signal in the direction in which the wavelength of said input fundamental wave coincides with the fundamental wavelength at which the wavelength conversion efficiency of said non-linear optical element is the maximum, whereby the oscillation wavelength of said semi-conductor laser and hence the output fundamental wavelength of said external frequency modulator are made equal to the wavelength at which the wavelength conversion efficiency of said non-linear optical element is the maximum. A wavelength converter according to claim 6, wherein said non-linear optical element is a light harmonic generating element. A wavelength converter according to claim 6, wherein said non-linear optical element has a quasi-phase matching structure. A wavelength converter according to claim 6, wherein said non-linear optical element is a Cherenkov-type non-linear optical element. A wavelength converter according to claim 6, wherein said non-linear optical element is made of LiTa x Nb 1-x O 3 .

Inventors: OGAWA TSUYOSHI, OGAWA, TSUYOSHI,, Ogawa, Tsuyoshi, c/o SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: H01S3
IPC codes: H01S5, G02F1, H01S3