CN103346473A - Method for narrowing line width of semiconductor laser by means of improved phase modulation heterodyning technology based on electric feedback - Google Patents
Method for narrowing line width of semiconductor laser by means of improved phase modulation heterodyning technology based on electric feedback Download PDFInfo
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- CN103346473A CN103346473A CN 201310273905 CN201310273905A CN103346473A CN 103346473 A CN103346473 A CN 103346473A CN 201310273905 CN201310273905 CN 201310273905 CN 201310273905 A CN201310273905 A CN 201310273905A CN 103346473 A CN103346473 A CN 103346473A
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Abstract
The invention discloses a method for narrowing the line width of a semiconductor laser by means of an improved phase modulation heterodyning technology based on electric feedback. The method for narrowing the line width comprises the steps: an output signal of the laser is connected to a frequency shifting MZ interferometer through an acoustic optical modulator, the frequency noise of the laser is converted into phase noise, coherent demodulation is carried out by a radio-frequency signal with controllable frequency deviation, then, frequency correction signals are obtained through amplification and filtering, a part of the frequency correction signals enter a driving current of the laser, the other part of the frequency correction signals are connected to an electric input end of the acoustic optical modulator behind a photoelectric detector, the frequency of the laser is corrected, the phase noise is reduced, and therefore the aim of narrowing the line width is achieved. The method for narrowing the line width has the advantages that the device is simple and easy to achieve, and a high-fineness FP cavity is not required; the line width is narrowed, and the frequency can be modulated at the same time; a high-quality light source is provided for a coherent light communication and high-resolution spectrograph; the requirements for the frequency of the laser do not exist, and therefore narrowing of the line width of the laser in various wavebands can be achieved.
Description
Technical field
The invention belongs to the semiconductor laser live width and press narrow field, particularly a kind of method of pressing the narrow semiconductor laser linewidth based on the modified model phase place modulation heterodyne technology of electricity feedback.
Background technology
In the ideal case, the light signal of semiconductor laser output single longitudinal mode, purity height, monochromaticjty are good.Nineteen eighty-two, the theory of the longitudinal mode semiconductor laser live width broadening that Henry proposes has been illustrated the wideer reason of semiconductor laser live width.Semiconductor laser in the reality is because the spontaneous radiation of internal gain medium causes certain live width, and the variation of carrier concentration has simultaneously also caused the broadening of live width.Therefore both at home and abroad at the live width of how to press narrow laser, carried out a large amount of research and proposed some live widths pressing narrow method.
The light feedback technique is used to press the live width of narrow semiconductor laser the earliest, is divided into two kinds of methods of exocoel light feedback and harmonic light feedback.Exocoel light feedback is directly a part of output signal of laser to be returned resonant cavity, reaches the purpose of pressing narrow linewidth by the length of regulating resonant cavity.The harmonic light feedback is at first passed through filtering with the signal of laser, and then feeds back to resonant cavity, can realize the frequency of laser is automatically become locked to optical resonator.But on the whole, the light feedback needs bigger optical feedback cavity, and has the moding phenomenon, and the stability of a system is poor.
In the past few decades, the researcher adopts atom or molecule saturated absorbing body to press narrow laser linewidth as the reference frequency, has reached certain effect, makes response but can't float to frequency fast; Adopt phase modulated light heterodyne (PDH) technology that the frequency of laser is locked onto on the resonance frequency in overstable F-P chamber, thereby obtain the live width and 10 of KHz magnitude
-15Frequency stability.Yet this method exists some restraining factors: the collimation height that requires free space optical elements; Strict Polarization Control; The pattern matching in space; Higher to the parameter request of resonant cavity simultaneously, overstable resonant cavity is bulky, frangible and cost height not only; And be not easy to realize big swept frequency range.
Summary of the invention
The present invention seeks to overcome the prior art above shortcomings, propose a kind of modified model phase place modulation heterodyne technology that feeds back based on electricity and press narrow semiconductor laser method, this live width presses narrow method can solve frequency discriminator operating difficulties, cost height in the existing pressure narrow linewidth method, be easy to generate the not problem of continuously-tuning of frequency drift, frequency.
Technical solution of the present invention:
A kind of method of pressing the narrow semiconductor laser linewidth based on the modified model phase place modulation heterodyne technology of electricity feedback, this method comprises: the output light of semiconductor laser is connected to coupler through acousto-optic modulator, the part of coupler output is connected to spectrometer, another part is sent into frequency discriminator, and frequency discriminator is made of MZ interferometer, acousto-optic frequency shifters and photodetector; The detected photo-signal of photodetector and local oscillation signal mixing, carry out coherent demodulation and obtain an intermediate frequency error signal, this signal is sent into the drive circuit of laser again through a part after amplification, the filtering, another part is sent into the electrical input of acousto-optic modulator behind the laser, frequency signal to laser is revised, thereby reduce laser phase noise, stabilized frequency, and then obtain live width and press laser signal after narrow.
The principle of the inventive method is seen Fig. 1, and wherein, solid line represents light signal, and thick dashed line represents radiofrequency signal, and fine dotted line represents low frequency signal.
The live width of described semiconductor laser is the laser signal of GHz magnitude.
Acousto-optic modulator driving signal frequency behind the described laser is 40MHz.
The radiofrequency signal source frequency of the acousto-optic modulator that inserts in the described MZ interferometer is 70MHz.
The length of described fiber delay line is 26nm.
Described local oscillation signal frequency as coherent demodulation is the twice of radiofrequency signal source frequency, is 140MHz, and controlled frequency deviation is
Described laser signal is observed by spectrometer.
Modified model phase place modulation heterodyne technology based on the electricity feedback presses the step of narrow semiconductor laser linewidth as follows:
1) driven with current sources semiconductor laser work.
2) frequency is the signal driving acousto-optic modulator work of 40MHz, and first-order diffraction light is as output signal.
3) first-order diffraction light by shift frequency MZ interferometer will with photodetector after change the frequency variation signal of laser into phase-changing signal.
4) by the adjustable local oscillation signal of frequency deviation the output signal of photodetector is carried out coherent demodulation, obtain the phase error signal of laser.This error signal is proportional to
,
Be the frequency deviation of local oscillation signal, wherein
Behind photodetector, obtain the phase changing capacity of laser, i.e. error signal.
5) this error signal is divided into the injection electric current that acts on laser and the acousto-optic modulator behind the laser through amplifying, obtain after the filtering frequency corrected signal, and the frequency of stable laser reduces the phase noise of laser, presses narrow linewidth.
6) realize interior continuous tuning to laser frequency among a small circle by the frequency deviation of regulating local oscillation signal.The laser frequency tuning range is
,
Time-delay for MZ.
Know-why of the present invention:
The light signal that laser sends obtains frequency error signal by shift frequency MZ interferometer after phase shift, and by coherent demodulation, amplification and filtered frequency corrected signal to laser, acousto-optic modulator after acting on the drive circuit of laser respectively and being positioned at laser, the frequency change of compensate for laser, thereby the frequency of stable laser, reduce phase noise, press narrow laser linewidth.
Advantage of the present invention and beneficial effect:
This live width presses that narrow square law device simply, convenient is implemented, system cost is low; Adopt shift frequency MZ interferometer to greatly reduce the complexity of frequency discrimination device, realize easily, and can realize tuning to laser frequency; The frequency corrected signal is fed back through two branch roads, improved the response speed of system, can handle phase jitter fast; This method is suitable for too to the laser of other types, can realize the pressure of various long wavelength laser live widths narrow.
The present invention is further described with implementing profit below in conjunction with accompanying drawing.
Description of drawings
Fig. 1 is the theory diagram of pressing the method for narrow semiconductor laser linewidth based on the modified model phase place modulation heterodyne technology of electricity feedback.
Fig. 2 is the theory diagram of MZ interferometer.
Fig. 3 is the schematic diagram of shift frequency MZ interferometer and demodulation.
Fig. 4 is the frequency of shift frequency MZ interferometer and the relation of amplitude.
Fig. 5 is active proportional-integral filter circuit diagram.
Fig. 6 is the output spectrum of laser when adding the frequency stabilization driving power.
Fig. 7 presses the output spectrum of laser after the narrow method for adopting live width.
Embodiment
Embodiment
A kind of modified model phase place based on the electricity feedback is modulated the method that heterodyne technology is pressed the narrow semiconductor laser linewidth, and principle is seen Fig. 1, and this method comprises:
The output of laser is connected to coupler through acousto-optic modulator, and the part of coupler output is connected to spectrometer, and another part is sent into frequency discriminator (being made of MZ interferometer, acousto-optic frequency shifters and photodetector), sees the frame of broken lines shown in the label 1 among Fig. 1.Acousto-optic frequency shifters is driven by the radio frequency source of 70MHz, radiofrequency signal is carried out coherent demodulation as local oscillation signal to the output signal of photodetector after 2 frequencys multiplication, thereby the frequency change that obtains laser is transformed into phase place and changes, after amplification, filtering, be connected respectively to the drive circuit of laser and the some input of acousto-optic modulator, revise the frequency of laser, reach the purpose of pressing narrow linewidth.Among Fig. 1, realize representing light signal, thick dashed line represents radiofrequency signal, and fine dotted line represents low frequency signal.
The electric field of laser signal can be expressed as:
, wherein
Be the amplitude of laser,
Be the laser angular frequency,
Be the laser initial phase.The two-way beat signal of light signal behind shift frequency MZ interferometer carries out mixing at photodetector, and the current signal after filtering DC component and the high fdrequency component can be expressed as:
, wherein
Be electrical signal amplitude,
Be the shift frequency amount of acousto-optic frequency shifters,
Be the laser angular frequency,
Be the laser initial phase,
It is time-delay
The phase place of back laser.The radio frequency signals drive that acousto-optic frequency shifters is 70MHz by a frequency, according to the characteristic of acoustooptic modulation, so be 140MHz as the local oscillation signal frequency of coherent demodulation, frequency deviation is
, the electric field expression formula of local oscillation signal can be write as
, wherein
Be the local oscillation signal amplitude,
Be the local oscillation signal frequency deviation,
Be the shift frequency amount of acousto-optic frequency shifters, the restituted signal expression formula by frequency mixer output after the coherent demodulation is:
, wherein
Be the intermediate-freuqncy signal amplitude, make the detected error phase of MZ interferometer
, therefore the low frequency aberration signal that obtains is proportional to
The frequency error signal that demodulates is through the injection electric current that amplifies, a part acts on laser after the low-pass filtering, revises the frequency of laser on a large scale, and another part acts on the acousto-optic modulator behind the laser, and fast-changing laser frequency is revised.In the bandwidth of feedback loop, reduce the frequency noise of laser, and then obtain the narrow linewidth semiconductor laser of low phase noise.When loop is in the closed loop state, can realize continuous tuning to laser frequency by the adjusting to local oscillation signal, so the tuning range of laser frequency can be expressed as:
Fig. 2 is the structure of MZ interferometer, wherein represents first three-dB coupler No. 2, represents monomode fiber No. 3, represents second three-dB coupler No. 4.Carry out square-law detection by photodetector behind the MZ interferometer of light signal through two arm quadratures, output signal is
So,
,
Be positive integer.The angular frequency of laser
With frequency
Between the pass be
So the frequency of laser is locked onto the point of series of discrete
On, c is the light velocity
,
Length for delay line.
Fig. 3 is the schematic diagram of shift frequency MZ interferometer and demodulation, wherein represents first three-dB coupler No. 5, represents second three-dB coupler No. 6, represents radio-frequency signal source No. 7, represents mixing No. 8.This part is the core that live width is pressed narrow method.In order to obtain good frequency error signal, time-delay arm fiber lengths is 2km(when specifically implementing
), the driving frequency of acousto-optic frequency shifters is 70MHz, realizes moving of 140MHz frequency, and the two-way beat signal of last interferometer reference arm and signal arm is that the photodetector of 0.9A/W is restrained detection in responsiveness, and the frequency of exporting heterodyne signal is
, wherein
Be shift frequency amount 140MHz,
Phase noise for laser.By frequency be
Local oscillation signal heterodyne signal is carried out coherent demodulation,
Frequency offset for local oscillator.
Fig. 4 is the normalized frequency of shift frequency MZ interferometer and the relation between the amplitude.The transfer function of shift frequency MZ interferometer is:
,
Be Fourier frequency,
Time-delay for the MZ interferometer.Fig. 4 illustrates that time-delay is more long, and the inspection product sensitivity of MZ interferometer is just more high.
What loop filter adopted is proportional-integral filter, and proportional-integral filter is a kind of loop filter commonly used, belongs to linear low-pass filters, is divided into two kinds of active proportional-integral filter and passive proportional-integral filter.Active proportional-integral filter has two independently adjustable parameters, has the leading characteristic that lags behind, high fdrequency component and noise in not only can the filtering error voltage, and can reach the purpose of stablizing loop.Therefore adopt active proportional-integral filter, circuit diagram as shown in Figure 5.The transfer function of filter can be expressed as:
, wherein,
,
, wherein
Be multiplication factor.Pass through controlling resistance
And electric capacity
Realize low-pass filtering, the output frequency corrected signal.
Fig. 6 result shows, adds constant-current source and drives, and laser output spectrum when adding the method for pressing narrow linewidth, this moment, three dB bandwidth was 0.5nm.
Fig. 7 result shows, adds the method for pressing narrow linewidth when adding constant-current source, the output spectrum to laser, this moment, live width was 0.02nm, laser is pressed narrow 25 times.
Claims (6)
1. the modified model phase place based on the electricity feedback is modulated the method that heterodyne technology is pressed the narrow semiconductor laser linewidth, it is characterized in that: the output light of semiconductor laser is connected to coupler through acousto-optic modulator, the part of coupler output is connected to spectrometer, another part is sent into frequency discriminator, and frequency discriminator is made of MZ interferometer, acousto-optic frequency shifters and photodetector; The detected photo-signal of photodetector and local oscillation signal mixing, carry out coherent demodulation and obtain an intermediate frequency error signal, this signal is sent into the drive circuit of laser again through a part after amplification, the filtering, another part is sent into the electrical input of acousto-optic modulator behind the laser, frequency signal to laser is revised, thereby reduce laser phase noise, stabilized frequency, and then obtain live width and press laser signal after narrow.
2. modulate the method that heterodyne technology is pressed the narrow semiconductor laser linewidth according to the described modified model phase place based on the electricity feedback of claim 1, it is characterized in that: the live width of the semiconductor laser of research is in the GHz magnitude.
3. modulate the method that heterodyne technology is pressed the narrow semiconductor laser linewidth according to the described modified model phase place based on the electricity feedback of claim 1, it is characterized in that: the driving frequency of the acousto-optic modulator behind the laser is 40MHz.
4. modulate the method that heterodyne technology is pressed the narrow semiconductor laser linewidth according to the described modified model phase place based on the electricity feedback of claim 1, it is characterized in that: the length of fiber delay line is 26nm.
5. according to the described method of pressing the narrow semiconductor laser linewidth based on the modified model phase place modulation heterodyne technology of electricity feedback of claim 1, it is characterized in that: according to the acoustooptic modulation effect, the drive source of acousto-optic frequency shifters is 70MHz, obtains the local oscillation signal of coherent demodulation by two frequencys multiplication.
6. modulate the method that heterodyne technology is pressed the narrow semiconductor laser linewidth according to the described modified model phase place based on the electricity feedback of claim 1, it is characterized in that: the output laser signal is observed by spectrometer.
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| CN107086428A (en) * | 2017-06-08 | 2017-08-22 | 中国电子科技集团公司第三十四研究所 | The narrow linewidth fiber pulse laser and its application method of a kind of high-peak power |
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| CN107086428B (en) * | 2017-06-08 | 2023-06-09 | 中国电子科技集团公司第三十四研究所 | High-peak-power narrow linewidth fiber pulse laser and application method thereof |
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| CN111256955A (en) * | 2020-01-19 | 2020-06-09 | 北京理工大学 | Optical coherent system performance evaluation method based on laser frequency noise spectrum |
| CN112929087A (en) * | 2021-01-28 | 2021-06-08 | 中国科学院半导体研究所 | Image frequency suppression mixing transmission method and device |
| CN113178773A (en) * | 2021-04-16 | 2021-07-27 | 山西大学 | Method for dynamically controlling feedback phase in optical feedback lock cavity technology |
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Application publication date: 20131009 |