WO2013189107A1 - 一种宽带连续可调谐激光器 - Google Patents

一种宽带连续可调谐激光器 Download PDF

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Publication number
WO2013189107A1
WO2013189107A1 PCT/CN2012/078332 CN2012078332W WO2013189107A1 WO 2013189107 A1 WO2013189107 A1 WO 2013189107A1 CN 2012078332 W CN2012078332 W CN 2012078332W WO 2013189107 A1 WO2013189107 A1 WO 2013189107A1
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laser
tunable
light
fabry
cavity
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PCT/CN2012/078332
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English (en)
French (fr)
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高培良
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天津奇谱光电技术有限公司
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Publication of WO2013189107A1 publication Critical patent/WO2013189107A1/zh
Priority to US14/339,605 priority Critical patent/US9257811B2/en

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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1062Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using a controlled passive interferometer, e.g. a Fabry-Perot etalon
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1068Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using an acousto-optical device
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00Semiconductor lasers
    • H01S5/10Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14External cavity lasers
    • H01S5/141External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00Semiconductor lasers
    • H01S5/10Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14External cavity lasers
    • H01S5/141External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • H01S5/142External cavity lasers using a wavelength selective device, e.g. a grating or etalon which comprises an additional resonator
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08Construction or shape of optical resonators or components thereof
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/106Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
    • H01S3/1065Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using liquid crystals
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01SDEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106Mode locking
    • H01S3/1109Active mode locking

Definitions

  • the invention belongs to the field of optoelectronics, and in particular to an external cavity wideband continuous tunable laser using a tunable Fabry-Perot filter and a tunable acousto-optic filter.
  • the commonly used tuning techniques mainly have the following methods: 1. Tuning by rotating the grating by a precision stepping motor, the problems are as follows: First, to achieve the precision of the optical frequency Tuning, the stepping accuracy and repeatability of the stepping motor are very high, so the manufacturing cost is relatively high; Second, due to the use of the stepping motor, it is not easy to achieve miniaturization; Third, the working stability in poor working environment is relatively poor. Especially, the ability to resist various types of mechanical vibrations is relatively poor. Therefore, tunable lasers using this technology are only suitable for use in laboratory work environments. 2. Tuning with tunable acousto-optic filter has the advantages of fast tuning speed, no mechanical moving parts, and miniaturization.
  • the disadvantage is that the tunable acousto-optic filter has a wide filtering bandwidth, so the tuning precision of the laser is not High, therefore, tunable lasers that simply use this technique are difficult to achieve precision continuous tunability and are only suitable for applications where tuning accuracy and output bandwidth are not high. 3.
  • the disadvantage is speed comparison.
  • the temperature drift coefficient of the optical filter component is 0.02 nm / kW, the required optical spectrum range is 20 nm, and the temperature adjustment range is 100 degrees. This is very difficult to implement in practical applications.
  • a broadband continuous tunable laser comprising a first laser cavity mirror sequentially mounted in a laser cavity, a laser gain medium, an intracavity collimating lens, and a source optical phase modulator, a tunable acousto-optic filter, further comprising an intracavity mirror, a tunable Fabry-Perot filter, a second laser cavity mirror, and a laser drive control system;
  • the intracavity collimating lens Aligning light emitted by the laser gain medium and entering the tunable acousto-optic filter at a Bragg angle;
  • the intracavity mirror is placed at a position such that the tunable acousto-optic filter is first The diffracted first-order diffracted light is again reflected into the tunable acousto-optic filter to obtain a second diffraction, and the optical frequency Doppler caused by the first diffraction can be eliminated.
  • the first laser cavity mirror, the intracavity mirror and the second laser cavity mirror are one of the following types of mirrors: a flat mirror, a concave mirror and a convex mirror, having partial or 100% reflectivity and
  • the laser gain medium has the same spectral range;
  • the first laser cavity mirror is a multilayer dielectric film directly plated on one end face of the laser gain medium;
  • the intracavity mirror is either a reflective grating or other A mirror with light dispersion.
  • the laser gain medium is a broadband laser gain medium.
  • the active optical phase modulator can be one of the following types: an electro-optical phase modulator, an acousto-optic phase modulator, a magneto-optical phase modulator, or some combination of the above-described phase modulators.
  • the tunable acousto-optic filter includes an acousto-optic crystal and an electro-acoustic transducer disposed on the acousto-optic crystal.
  • the tunable Fabry-Perot filter is formed by sequentially connecting the first mirror, the liquid crystal module and the second mirror; the intrinsic free spectral range in the absence of an applied electric field is between 10 GHz and 400 GHz, A Fabry-Perot cavity is formed by the first light-passing surface of the first mirror and the second light-passing surface of the second mirror.
  • the first light-passing surface of the first mirror is provided with a high-reflectivity multilayer dielectric film; and the second light-passing surface of the first mirror is sequentially provided with an optical anti-reflection film, a transparent electrode and an electrode Isolation membrane
  • the material of the liquid crystal module is a nematic liquid crystal material having a thickness of several micrometers to ten micrometers and having an optical phase retardation of at least 2 ⁇ for linearly polarized light in a certain direction driven by an applied electric field, and Has the same spectral range as the laser gain medium.
  • the first layer of the first light-passing surface of the second mirror is provided with a film of a non-conductive material covering a portion other than the light-passing aperture, and has a thickness of several micrometers to ten micrometers;
  • the second reflection a second layer of the first light-passing surface of the mirror is provided with an optical anti-reflection film;
  • a third layer of the first light-passing surface of the second mirror is provided with a transparent electrode;
  • a second layer of the second mirror is disposed
  • the light-passing surfaces are provided with a high-reflectivity multilayer dielectric film and have the same reflectivity as the first light-passing surface of the first mirror.
  • the laser frequency and resonant mode locker includes: a first Fabry-Perot etalon disposed on an optical path of the second-order diffracted zero-order light and located outside a cavity of the laser cavity; a first photodetecting device placed in said After the first Fabry-Perot etalon, used to detect the power of light transmitted from the first Fabry-Perot etalon; the second Fabry-Perot etalon, placed An optical path of zero-order light that is reflected by the second laser cavity mirror and diffracted by the tunable acousto-optic filter is located outside the cavity of the laser cavity; a second photodetecting device is placed in the chamber After the second Fabry-Perot etalon is used, it is used to detect the power of light transmitted from the second Fabry-Perot etalon.
  • the first Fabry-Perot standard has the same optical spectral range as the laser gain medium, the sharpness coefficient such that the difference between the maximum and minimum values of the transmitted light intensity is greater than 3 dB; and the free spectral range is Four times the intrinsic free spectral range of the tunable Fabry-Perot filter; a certain frequency of its light transmission peak and a certain peak of the light transmission of the tunable Fabry-Perot filter The frequency is the same.
  • the second Fabry-Perot standard has the same optical spectral range as the laser gain medium and has the same sharpness and free spectrum as the first Fabry-Perot standard The range; the difference between the light transmission peak and the first Fabry-Perot standard light transmission peak is equal to the intrinsic free spectral range of the tunable Fabry-Perot filter.
  • the laser power locker includes a photodetecting device located outside the cavity of the laser cavity, placed a second time reflected by the second laser cavity mirror and diffracted by the tunable acousto-optic filter The zero-order optical path of the diffracted light.
  • the laser drive control system includes: a digital signal microprocessor (DSP) and embedded laser control software, four digital-to-analog conversion modules, a laser pump source, an active optical phase modulator drive source, An RF signal source, a tunable Fabry-Perot filter drive source, a semiconductor refrigeration device temperature control device, two analog to digital conversion modules, a laser frequency and resonant mode locking device, and a laser power locking device
  • DSP digital signal microprocessor
  • embedded laser control software four digital-to-analog conversion modules
  • a laser pump source an active optical phase modulator drive source
  • An RF signal source a tunable Fabry-Perot filter drive source
  • a semiconductor refrigeration device temperature control device two analog to digital conversion modules
  • a laser frequency and resonant mode locking device a laser power locking device
  • the digital signal microprocessor directly drives and controls the temperature control device, and controls the laser pump source, the active optical phase modulator driving source, the RF signal source, and the tunable Fabry by
  • a driving source of the Perot filter driving the laser frequency and the resonant mode locking device and the laser power locking device by two analog-to-digital conversion modules to realize a laser gain medium, an active optical phase modulator, and a tunable Acousto-optic filter and tunable Fabry-Perot filter, temperature control device, laser frequency and resonant mode lock
  • the laser system is designed to be tunable by the phase modulation of the liquid crystal and the thin size (about 10 microns) in the light passing direction, combined with the traditional Fabry-Perot etalon technology.
  • the Fabry-Perot filter combined with a tunable acousto-optic filter, not only reduces the narrow filtering bandwidth requirements for tunable acousto-optic filters, but also enables lasers in a wide spectral range. Fast and precise tuning of the frequency ensures a large tuning spectral range and a narrow laser output spectrum.
  • the laser utilizes a laser cavity internal mirror to not only achieve secondary diffraction of the beam in the laser cavity, but also overcomes the optical frequency drift typically produced by a tunable acousto-optic filter with a single acousto-optic crystal and a single transducer. , making the output spectrum of the laser more stable, and according to different laser gain medium and application requirements, different mirror types and two laser end face mirrors form different types of laser cavity types, and simultaneously adjust the laser cavity The distance between the mirror and the tunable acousto-optic filter can adjust the loss of the laser cavity.
  • the laser uses three tunable acousto-optic filters to generate three zero-order diffracted beams in the laser cavity.
  • the laser light is realized without adding components in the laser cavity and without inserting a spectroscopic device on the output optical path. Frequency and resonant mode and monitoring and locking of optical power. Not only does it reduce the cost of the laser, but it also improves the performance of the laser.
  • the invention has reasonable design and can realize stable laser output with optical frequency tuning precision less than 1 GHz and narrow spectrum bandwidth in a wide spectral range, has no mechanical moving parts, stable and reliable performance, low cost, small size, easy installation and production, etc.
  • Figure 1 is a schematic diagram of a conventional tunable acousto-optic filter
  • FIG. 2 is a schematic diagram of a tunable laser 100 based on a tunable acousto-optic filter
  • Figure 3-1 is a wave vector diagram of the primary diffracted incident beam, the acoustic field, and the diffracted beam in an acousto-optic crystal
  • Figure 3-2 shows the wave-vector relationship of the incident diffracted beam, the acoustic field, and the diffracted beam in an acousto-optic crystal
  • Figure 4 is a schematic view of a conventional Fabry-Perot light etalon
  • Figure 5 is a schematic diagram of a tunable Fabry-Perot filter including a liquid crystal module
  • FIG. 6 is a schematic diagram showing a relationship between an optical phase and an electric field of a liquid crystal module under an external electric field
  • Figure 7 is a schematic diagram of the transmission spectrum of a conventional Fabry-Perot optical etalon
  • Figure 8 is a schematic diagram of the transmission spectrum of a tunable Fabry-Perot filter
  • Figure 9 is a schematic structural view of the present invention.
  • Figure 10 is a schematic block diagram of a laser drive control system of the present invention.
  • Figure 11 is a transmission spectrum of two Fabry-Perot etalons used to lock the laser frequency and resonant mode.
  • FIG. 1 is a schematic structural view of a conventional tunable acousto-optic filter.
  • the tunable acousto-optic filter has only one transducer, and the incident beam 2 is incident on the acousto-optic crystal 30 at a Bragg angle to generate a zero-order diffracted beam 3 And the first-order diffracted beam 4.
  • the working principle of the acousto-optic filter is based on a phenomenon of Bragg diffraction.
  • Bragg diffraction involves the interaction of photons (quantum of light energy) and phonons (quantum of acoustic energy). In this interaction process, both energy and momentum It is conserved.
  • the Acousto-Optical Tunable Filter is a solid-state, electrically tunable, bandpass optical filter. Compared to traditional technologies, AOTF provides continuous, fast adjustment and narrow spectral bandwidth.
  • AOTF provides continuous, fast adjustment and narrow spectral bandwidth.
  • FIG. 2 shows the structure of a laser 100.
  • mirrors 45, 50, and 57 constitute the resonant cavity of the laser.
  • the broadband fluorescent light beam 48 emitted by the laser gain medium 47 is collimated by the intracavity collimating lens 49 to enter the acousto-optic crystal 30 in the tunable acousto-optic filter at a Bragg angle, and the first-order diffracted light is diffracted.
  • the light 4 is reflected by the intracavity mirror 50 and then enters the acousto-optic crystal 30 again at a Bragg angle.
  • the second-order diffracted first-order diffracted light 8 is reflected back into the laser cavity by the second laser cavity mirror 57, and is formed in the laser cavity.
  • Laser oscillation and amplification In this process, the beam 3 acts as the laser output beam due to its maximum energy and zero-drift after the second diffraction of the tunable acousto-optic filter, and the other zero-order diffracted beams 6, 7 and 9 act as laser cavity beams. Leak out of the laser cavity; beams 6, 7, and 9 can be used to monitor the optical power and wavelength in the laser cavity, etc., to avoid inserting other spectroscopic devices into the laser cavity or the output optical path to achieve such a function.
  • the laser 100 is compact, and the mirrors 45, 50, and 57 can use the full mirror to form the cavity of the laser, further reducing the loss of the laser cavity.
  • the acousto-optic tunable filter with a single crystal and a single transducer can be used. Eliminate the Doppler shift of the frequency.
  • the incident beam 2 is incident on the acousto-optic crystal 30 at a Bragg angle, producing a zero-order diffracted beam 3 and a first-order diffracted beam 4, which is reflected by the intracavity total reflection mirror 50 again.
  • the entry into the acousto-optic crystal 30 at a Bragg angle produces a zero-order diffracted beam 6 and a first-order diffracted beam 8.
  • the transducer is coupled to an RF signal source 10 that acts as a drive source for the tunable acousto-optic filter, provides RF energy, and adjusts the oscillation wavelength of the laser cavity by varying the RF frequency.
  • Figure 3-1 and Figure 3-2 show the wave vector relationship of incident light ( K i ), diffracted light ( K d ), and sound wave ( KS ).
  • K d diffracted light
  • KS sound wave
  • the acousto-optic crystals employed are anisotropic and have birefringence characteristics.
  • One of these materials is cerium oxide (Te02), which is widely used in such applications due to its high optical uniformity, low light absorption and high optical power capability in shear mode.
  • Other substances such as lithium niobate (LiNb03), gallium phosphide (GaP) and lead molybdate (PbMo04) are also frequently used in various acousto-optic devices. There are many factors that influence the selection of specific substances.
  • Figure 4 shows a schematic of a conventional Fabry-Perot optical etalon 43.
  • the material of the Fabry-Perot etalon 43 is generally optical glass such as fused silica or BK7 in the near-infrared and visible-light bands, assuming that the material has a refractive index n and both light-passing surfaces 41 and 42 are plated high.
  • the frequency broadband of the transmitted light is mainly related to the reflectance R. The higher the reflectance, the smaller the frequency broadband or the finesse. Transmitted light from Fabry-Perot etalon The spectrum is characterized by a very narrow bandwidth for each transmission spectrum, equal frequency spacing of the output spectrum and a very wide optical band width, as shown in Figure 7.
  • Liquid crystal materials generally used as photovoltaic devices have high resistivity. Therefore, it can be considered as an ideal dielectric material.
  • the liquid crystal has anisotropic dielectric properties and uniaxial symmetry due to the ordered orientation of the constituent molecules and the stretched morphology. Like a uniaxial crystal, the direction of the optical axis coincides with the alignment of the molecules.
  • an electric dipole is formed. Under the action of the moment formed by the electric dipole, the orientation of the liquid crystal molecules is turned to the direction of the electric field, and the direction of the optical axis of the liquid crystal can be changed by changing the strength of the electric field.
  • this characteristic of the liquid crystal can be utilized to fabricate an optical phase modulator, a tunable filter, or other optoelectronic devices such as an optical switch and a light intensity modulator.
  • the thickness of the liquid crystal film layer generally used as a photovoltaic device is about 10 ⁇ m.
  • the present invention has invented the tunable Fabry-Perot filter using this characteristic design.
  • FIG. 5 is a tunable Fabry-Perot filter designed to change the refractive index of linearly polarized light by an electric field.
  • the tunable Fabry-Perot filter 200 includes two sheets of optically transparent material 16 and 26, and a highly reflective multilayer dielectric film layer having a reflectivity R plated on the surfaces 18 and 29 of the two sheets of optically transparent material.
  • the two reflective film layers form a Fabry-Perot cavity; an antireflection film is disposed on the transmissive film layer and transparent electrode film layers 21 and 28 are disposed respectively, and a control signal 22 is formed between the two transparent electrode film layers.
  • the electric field is driven; a liquid crystal film layer 24 is disposed in a cavity formed between the two film layers of 21 and 28.
  • a Fabry-Perot filter can be fabricated in the intrinsic free spectral range (in the free spectral range of the filter without an applied electric field).
  • the effective refractive index n of the liquid crystal in the Fabry-Perot cavity is varied by an electric field to adjust the optical frequency v and the free spectral range (FSR) of the transmitted light of the Fabry-Perot filter.
  • the usual driving electric field is a square wave signal with a voltage of a few volts and a frequency of a few hertz to several kilohertz.
  • the light beam 15 incident on the filter 200 is a beam traveling in the z direction, and the polarization axis is linearly polarized light in the X direction, assuming that the refractive index of the optically transparent material is n, the two light passing surfaces 18 and 26 Both are highly reflective films, assuming a reflectivity of R and a thickness of D, then the filter 200
  • the free spectral range and transmitted light frequency are:
  • ⁇ 2/(2 ⁇ + ⁇ ), or expressed by frequency: Av c/(2nD+r), where c is the speed of light, and ⁇ represents the additional optical path produced by the liquid crystal under the applied electric field.
  • Figure 6 shows the phase of a light wave with a wavelength of 1550 nm driven by a square wave voltage.
  • the relationship of change. A maximum optical phase delay of about 6 ⁇ can be achieved.
  • the tunable Fabry-Perot filter 200 can obtain a tuning range of the transmitted optical frequency of about 100 GHz for linearly polarized light incident at near zero degrees, with an accuracy of less than 1 GHz.
  • the change in the band width of the free spectral range ⁇ and the transmitted light is much smaller.
  • Figure 8 shows a schematic of the transmission spectrum of a tunable Fabry-Perot filter.
  • the tunable Fabry-Perot filter 200 under the action of an applied electric field, can achieve a wide range of tuning of the peak frequency of the transmitted light without substantially changing the frequency broadband and free spectral range of the transmitted light. This feature is of great importance for the application of the tunable Fabry-Perot filter 200 in the present invention.
  • the structure of the broadband continuous tunable laser will be described in detail below.
  • the broadband continuous tunable laser 300 uses the tunable acousto-optic filter and the tunable Fabry-Perot filter 200 shown in FIG. It is: including a first laser cavity mirror 45 directly plated on the laser gain medium 47, a laser gain medium 47, an intracavity collimator lens 49, an active optical phase modulator 51, a tunable acousto-optic filter 100, and an intracavity a mirror 50, a tunable Fabry-Perot filter 200, and a second laser cavity mirror 57, wherein the first laser cavity mirror 45, the intracavity mirror 50 and the second laser cavity mirror 57 constitute a laser Resonant cavity.
  • the tunable Fabry-Perot filter 200 is disposed in the optical axis direction of the first-order diffracted beam of the second diffraction of the tunable acousto-optic filter, and the second laser cavity mirror is disposed in the tunable Fabry-Perot Behind the filter.
  • the wide-band laser gain medium including the first laser cavity mirror 45, the intracavity collimating lens, the active optical phase modulator, the tunable acousto-optic filter, and the intracavity mirror are placed such that only the tunable acousto-optic filter is passed
  • the second-order diffracted first-order diffracted ray can form a laser oscillation in the laser cavity.
  • Laser cavity mirrors typically have different reflectivities for different wavelengths or colors of light, and the reflectivity referred to herein is the reflectance corresponding to the spectral bandwidth at which the laser operates.
  • the first laser cavity mirror 45 can be a full mirror or a partial mirror depending on the situation. If the laser gain medium is a semiconductor gain medium, since there is generally a relatively large output dispersion angle, the intracavity collimator lens 49 is generally used when the laser gain medium is a semiconductor gain medium. When the laser gain medium is a gas, liquid or some solid medium, the intracavity collimating lens is generally not used, but a non-planar mirror is used to achieve a reasonable distribution of the beam in the cavity.
  • the intracavity collimating lens 49 not only collimates the light from the laser gain medium 47, but is also a collimating lens of the laser output beam.
  • the output beam 4 needs to be coupled into the fiber, and a collimating lens 49 is essential.
  • the broadband fluorescent light beam 48 emitted by the laser gain medium 47 is collimated by the intracavity collimating lens 49 and transmitted through the active optical phase modulator 51 to enter the tunable acousto-optic filter at a Bragg angle.
  • the acousto-optic crystal 30 in the device, the first-order diffracted first-order diffracted light 4 is reflected by the intracavity mirror 50 and then enters the sound and light again at the Bragg angle.
  • the crystal 30, the second-order diffracted first-order diffracted light 8 passes through the tunable Fabry-Perot filter 200 and is reflected back into the laser cavity by the second laser cavity mirror 57 to form a laser oscillation in the laser cavity.
  • the zero-order diffracted beams 3, 6, 7 and 9 act as the zero-order beam of the laser beam in the laser cavity outside the laser cavity; the beam 3 has the maximum energy and tunable sound during laser oscillation and amplification.
  • the wavelength is zero-drifted as a laser output beam. Beams 6, 7, and 9 can be used to monitor optical power and wavelength within the laser cavity. The details will be explained below.
  • the tunable acousto-optic filter is tunable because the frequency shift of the first-order diffracted first-order diffracted light and the second-order diffracted first-order diffracted light produce the opposite optical frequency shift.
  • the optical frequency offset caused by the laser output in the structure in laser 300 is zero.
  • a laser oscillation narrower than the primary diffraction bandwidth is formed in the laser cavity.
  • intracavity mirrors 50 and second laser cavity mirrors 57 can be used to form different laser cavities, and can also compensate the dispersion and divergence of the intracavity beam caused by the acousto-optic crystal diffraction, and reduce the laser.
  • adjusting the distance L between the intracavity mirror 50 and the acousto-optic crystal 30 see Fig. 2
  • the loss of the laser cavity can also be adjusted.
  • the optical frequency output by the tunable laser 300 is tuned by controlling the active optical phase modulator 51, driving the tunable acousto-optic filter RF signal source 10 and the tunable Fabry-Perot filter 200.
  • the RF frequency of the RF signal source 10 of the tunable acousto-optic filter is changed, and the frequency of the diffracted light in the laser cavity can be selected.
  • the active optical phase modulator 51 produces laser oscillation and amplification in a laser cavity by adjusting the phase of the beam within the cavity.
  • the filter 200 at this time is equivalent to a Fabry-Perot etalon, and the optical frequency of the tunable laser 300 is affected by the filter 200.
  • the output of the tunable laser 300 can only be tuned to one of the transmission spectra of the intrinsic transmission spectrum of the tunable Fabry-Perot filter 200.
  • the output spectrum of the laser 300 can be precisely tuned by adjusting the applied electric field of the tunable Fabry-Perot filter 200. In the precision tuning process, it is also necessary to fine tune the RF signals of the active optical phase modulator 51 and the RF signal source 10.
  • the tunable laser 300 can be realized over a wide spectral range.
  • the precision is continuously tunable.
  • the intrinsic transmission spectrum for filter 200 is a 100 GHz DWDM system that meets the International Telecommunications Union (ITU) standard.
  • the tunable laser 300 outputs the spectrum by adjusting the active optical phase modulator 51 and the tunable acousto-optic filter 100.
  • ITU Grid optical frequency requirements
  • the tunable Fabry-Perot filter 200 has an applied electric field, precise frequency tuning of the spectrum of transmitted light of 100 GHz can be achieved, and thus, it can be realized at two 100 GHz IU grid optical frequencies. Precision and continuous tuning between. Precision tuning of optical frequencies less than 1 GHz is generally possible.
  • the commonly used C frequency band (about 1530
  • the nano--1570 nm or L frequency band (about 1570 nm - 1610 nm) has an optical spectrum wideband of about 40 nm, and the tunable laser 300 is fully tunable in the C frequency band and/or the L frequency band.
  • the tunable Fabry-Perot filter 200 also determines the spectral width of the laser output light.
  • a Fabry-Perot filter with a high sharpness factor can compress the spectral bandwidth of the output beam and increase the side mode rejection ratio. Since the tuning process has negligible bandwidth effects on the transmission spectrum of the Fabry-Perot filter, the bandwidth of the output spectrum can be substantially uniform during the tuning process of the tunable laser 300.
  • the filter bandwidth of the acousto-optic tunable filter is such that the half-width of the intrinsic spectral bandwidth of the resonant mode within the cavity of the tunable laser 300 is such that no tunable Fabry is provided in the cavity.
  • the half width of the spectrum of the resonant mode at the time of the Perot filter 200 is less than the intrinsic free spectral range of the tunable Fabry-Perot filter 200 to ensure single mode operation within the cavity of the tunable laser 300. Since the tunable Fabry-Perot filter 200 does not substantially change the free spectral range during tuning, it is possible to continue to maintain single mode operation during the tuning of the laser 300.
  • the laser 300 may exhibit a transition of the resonant mode, that is, the laser jumps from a resonant mode to an adjacent mode, or other modes farther away, and main parameters such as frequency and optical power are also possible.
  • Frequency drift refers to the phenomenon of frequency drift that occurs when the laser operates without mode hopping. Generally, the frequency variation in frequency drift is relatively small. Therefore, in order to achieve stable operation of the laser, it is necessary to monitor and lock the above three parameters: resonant mode, frequency and optical power in real time.
  • the resonant mode and frequency of the laser 300 are monitored and locked in real time by a Fabry-Perot with low sharpness (low finesse) set on the zero-order diffracted lights 6 and 9 (Fig. 9) that leak out of the laser cavity.
  • the etalon 62 and 64, as well as the photodetecting devices 60 and 66 located after the etalon described above, and associated locking devices are implemented.
  • the Fabry-Perot etalon with low sharpness factor has a transmission spectrum that approximates a sinusoid.
  • Figures 10 and 11 show the transmission spectra of etalon 62 and 64 with the same sharpness factor, respectively.
  • the tunable Fabry-Perot filter 200 has an intrinsic free spectral range of FSR1 and Fabry -
  • the free spectral range of the Perot etalon 62 is four times the FSR1, and the first frequency ⁇ /1 of the transmission spectrum is the same as the first transmission frequency vl of the tunable Fabry-Perot filter 200, Fabry
  • the free spectral range of the Ri-Perot etalon 64 is also four times the FSR1, and the first frequency of the transmission spectrum is the same as the tunable Fabry-Perot filter 200 ⁇ /2 (or, Fabry-
  • the transmission peaks of the Perot etalon 62 and 64 differ by a quarter of the FSR1).
  • the optical power intensities obtained on the photodetecting devices 60 and 66 for light of different frequencies are different.
  • the maximum peak transmitted light intensity is 1, the minimum transmitted light intensity is zero, and the median value is 0.5
  • the light intensity values of the respective frequencies in the photodetecting devices 60 and 66 are listed in the following table: Light intensity (photoelectric Equipment 60) Light intensity (photovoltaic equipment 66) Vl 1 0.5
  • vlO 0.5 1 Due to the periodic nature of the transmission spectrum, only the light intensity values of the first ten frequencies are listed in the table. As can be seen from the above table, by comparing the two light intensity values of the optoelectronic device 60 and the optoelectronic device 66, it is possible to distinguish v1 from v2, v3, v4, ⁇ 6, ⁇ , v8; v2 is the same as vl, v3, v4, V5, v7, v8, v9, and so on. Vl, v5, v9, v2, v6, vlO are periodically repeated, and so on.
  • a light lookup table corresponding to each frequency is formed into a lookup table in the digital signal microprocessor, and it can be judged whether the laser 300 is subjected to mode hopping, and the laser drive control system can The resonant mode is locked. It should be pointed out that if the laser working in vl jumps to v5 or v9, or if the laser working in ⁇ /2 jumps to ⁇ /6 or vlO, the system will not be able to judge, and other modes can be analogized. . Therefore, the present invention can only make a judgment on the case of jumping to a non-periodic mode. In general, the mode hopping of the laser will jump to the adjacent mode. Therefore, the present invention can only judge the situation of the upper and/or lower three modes of the mode.
  • the direction of the mode hopping can be adjusted to the original resonant mode by changing the corresponding laser drive parameters, such as changing the RF frequency of the tunable acousto-optic modulator.
  • the present invention can only perform locking for the case where a mode jump occurs to the two nearest modes.
  • the FSR1 which is eight times and whose peak transmission frequency is doubled from each other, can be locked in the case of the mode transition to the nearest four modes.
  • frequency drift means that the frequency does not change by more than half of the two resonant mode frequency intervals (ie equal to
  • 0.5(v2-vl) o when the frequency drifts, since the light intensity transmitted through the Brill-Perot etalon 62 and 64 changes, it is detected on the photovoltaic devices 60 and 66. The light intensity will also change. Using changes in light intensity, changes in optical frequency can be detected and the frequency can be locked by the drive control system of laser 300, including changing the temperature, phase, and RF signal frequency of the cavity. As can be seen from Fig. 11, the odd modes of the laser modes vl, v3, v5, W, etc. are located in the central region of the transmittance curve below the Fig. 11, and the even modes of v2, v4, V6, V8 and the like are located above the top of Fig. 11.
  • the even mode can be monitored by detecting changes in the optical power of the optoelectronic device 60, the odd mode is monitored by changes in the optical power of the optoelectronic device 66, and the laser frequency is locked by the closed loop control loop of the drive control circuitry of the laser 300.
  • a photodetection device 68 is used to directly monitor the power variation of the laser 300 and to control the closed loop control loop of the laser through the laser and to lock the laser output power by changing parameters such as the pump power of the laser.
  • the drive control circuit system of the above tunable laser 300 is as shown in FIG.
  • the drive control circuitry includes: a digital signal microprocessor (DSP) 120 with an embedded software program, four digital to analog conversion (D/A) devices 110, 112, 113, and 115 for driving control of the laser gain, respectively.
  • DSP digital signal microprocessor
  • D/A digital to analog conversion
  • Laser pump source 109 of medium 47 phase modulator drive source 111 of active optical phase modulator, RF signal source 10 of tunable acousto-optic filter 100, Fabry of tunable Fabry-Perot filter
  • Perot filter drive source 114 two analog to digital conversion (A/D) devices 117 and 119, for driving the control laser frequency and resonant mode locking device 116 and laser power locking device 118, respectively.
  • A/D analog to digital conversion
  • the digital signal micro-controller 120 directly drives and controls the temperature control device 70, and can also receive an external command and separately control the laser pump source, the active optical phase modulator driving source, and the radio frequency signal source through four digital-to-analog conversion modules.
  • a tunable Fabry-Perot filter driving source driving the laser frequency and the resonant mode locking device and the laser power locking device by two analog-to-digital conversion modules to realize a laser gain medium, active light Phase modulator, tunable acousto-optic filter and tunable Fabry-Perot filter, temperature control device, laser frequency and resonant mode locking device, laser power locking device drive control, and optical frequency tuning and locking And the control and locking function of the output optical power.

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Abstract

本发明涉及一种宽带连续可调谐激光器,包括依次安装在激光腔内的第一激光腔反射镜、激光增益介质、腔内准直透镜、有源光相位调制器、可调谐声光滤波器,还包括腔内反射镜,使第一次衍射的一级衍射光被再次反射到可调谐声光滤波器中,从而使腔内光束得到第二次衍射,消除由可调谐声光滤波器第一次衍射所造成的激光频率多普勒漂移,并在第二次衍射的一级衍射光的光轴方向上设置可调谐法布里——珀罗滤波器,在可调谐法布里——珀罗滤波器后设置第二激光腔反射镜。本发明可实现在宽频谱范围内光频率调谐精度小于1GHz和窄频谱带宽的稳定激光输出,具有无机械移动部件、性能稳定可靠、成本低、尺寸小、易于安装及生产等特点。

Description

一种宽带连续可调谐激光器 技术领域
本发明属于光电领域,尤其是一种采用可调谐法布里 -珀罗滤波器和可调谐声光滤波器 的外腔式宽带连续可调谐激光器。
背景技术
在外腔式宽带连续可调谐激光器中, 常用的调谐技术主要有以下方式: 1、 通过精密 步进马达带动光栅的旋转来进行调谐的, 其存在的问题体现在: 一是为实现光频率的精密 调谐, 对步进马达的步进精度和重复性要求很高, 因此制造成本比较高; 二是由于采用步 进马达, 不易做到小型化; 三是在恶劣工作环境下的工作稳定性比较差, 特别是抗各类 机械振动的能力比较差, 因此, 采用该技术的可调谐激光器只适合用于实验室工作环境中 使用。 2、 利用可调谐声光滤波器进行调谐, 其优点是调谐速度快, 没有机械移动部件, 可以做到小型化, 缺点是可调谐声光滤波器的滤波带宽比较宽, 使得激光器的调谐精度不 高, 因此, 单纯采用这种技术的可调谐激光器很难做到精密连续可调谐, 只适合用于对调 谐精度和输出带宽不高的应用中。 3、 利用光栅或激光谐振腔中的其他光学滤波器件, 如 光学标准具等透射光频率随温度漂移的特点进行调谐, 其优点是调谐精度高和输出光的光 谱带宽比较窄, 缺点是速度比较慢, 特别是在要求调谐光谱范围宽的情况下, 这个缺点尤 为明显, 例如: 光学滤波器件的温度漂移系数是 0.02纳米 /度, 要求的光频谱范围是 20纳 米, 温度调节范围是 100度, 这在实际应用中是很难实现的。
发明内容
本发明的目的在于克服现有技术的不足, 提供一种调谐速度快、 精度高、 成本低、 尺 寸小且易于生产的宽带连续可调谐激光器。
本发明解决现有的技术问题是采取以下技术方案实现的: 一种宽带连续可调谐激光器, 包括依次安装在激光腔内的第一激光腔反射镜、 激光增 益介质、 腔内准直透镜、 有源光相位调制器、 可调谐声光滤波器, 还包括腔内反射镜、 可 调谐法布里-珀罗滤波器、第二激光腔反射镜及激光器驱动控制系统; 所述腔内准直透镜用 于将所述激光增益介质发出的光准直, 并以布拉格角进入所述可调谐声光滤波器; 所述腔 内反射镜放置的位置使得由所述可调谐声光滤波器第一次衍射的一级衍射光被再次反射 到所述可调谐声光滤波器中而得到第二次衍射, 并能消除由第一次衍射造成的光频率多普 勒漂移; 在第二次衍射的一级衍射光的光轴方向上设置所述可调谐法布里-珀罗滤波器, 在 所述可调谐法布里-珀罗滤波器后设置所述第二激光腔反射镜; 由所述的第一激光腔反射 镜、 所述的腔内反射镜和所述的第二激光腔反射镜构成了激光器谐振腔; 所述的第一激光 腔反射镜、 宽带激光增益介质、 腔内准直透镜、 有源光相位调制器、 可调谐声光滤波器、 可调谐法布里 -珀罗滤波器和腔内反射镜安放的位置使得只有经过所述可调谐声光滤波器 的第一次和第二次衍射的一级衍射光的光线才能在所述激光谐振腔内形成激光振荡; 所述 宽带连续可调谐激光器的输出光束和第一次衍射的零级光重合。
而且, 所述第一激光腔反射镜、 腔内反射镜和第二激光腔反射镜均为下列几种反射镜 之一: 平面镜, 凹面镜和凸面镜, 具有部分或 100%的反射率并与所述激光增益介质具有 相同光谱范围; 所述第一激光腔反射镜或者是直接镀在激光增益介质一个端面上的多层介 质膜; 所述腔内反射镜或者是一种反射型光栅或其他具有光色散的反射镜。
而且, 所述激光增益介质是一种宽带激光增益介质。
而且, 所述有源光相位调制器可以是下列几种类型之一: 电光相位调制器、 声光相位 调制器、 磁光相位调制器或上述几种相位调制器的某种组合。
而且, 所述可调谐声光滤波器包括一块声光晶体和设置在所述声光晶体上的一个电声 换能器。
而且, 所述可调谐法布里-珀罗滤波器由第一反射镜、液晶模块和第二反射镜依次连接 构成; 在没有外加电场时的本征自由光谱范围介于 10GHz和 400GHz之间, 由所述的第一 反射镜的第一个通光面和所述的第二反射镜的第二个通光面构成法布里-珀罗腔。
而且, 所述的第一反射镜的第一个通光面设置高反射率多层介质膜; 所述的第一反射 镜的第二个通光面依次设置光学增透膜、 透明电极和电极隔离膜;
而且, 所述液晶模块的材料是一种向列相型液晶材料,厚度为几微米到十几微米, 在外 加电场的驱动下对某一方向的线偏振光具有至少 2π 的光相位延迟, 并具有与激光增益介 质相同的光谱范围。
而且, 所述的第二反射镜的第一个通光面的第一层设置非导电材料薄膜, 覆盖除通光 孔径以外的部分, 厚度为几微米到十几微米; 所述的第二反射镜的第一个通光面的第二层 设置光学增透膜; 所述的第二反射镜的第一个通光面的第三层设置透明电极; 所述的第二 反射镜的第二个通光面设置高反射率多层介质膜并与所述的第一反射镜的第一个通光面 具有相同的反射率。
而且, 所述激光频率和谐振模锁定器包括: 第一个法布里-珀罗标准具, 放置在所述第 二次衍射的零级光的光路上并位于所述激光腔的腔外; 第一个光电探测装置, 放置在所述 第一个法布里-珀罗标准具后, 用于检测从所述第一个法布里-珀罗标准具透过的光的功率; 第二个法布里-珀罗标准具,放置在被所述第二激光腔反射镜反射后并经所述可调谐声光滤 波器衍射的零级光的光路上, 位于所述激光腔的腔外; 第二个光电探测装置, 放置在所述 第二个法布里-珀罗标准具后, 用于检测从所述第二个法布里-珀罗标准具透过的光的功率。
而且, 所述第一个法布里-珀罗标准具有与所述激光增益介质相同的光频谱范围, 其锐 度系数使得透射光强最大值和最小值的差大于 3dB;其自由光谱范围是所述可调谐法布里- 珀罗滤波器的本征自由光谱范围的四倍; 其光透射峰值的某一频率与所述可调谐法布里- 珀罗滤波器的光透射峰值的某一频率相同。
而且, 所述第二个法布里-珀罗标准具有与所述激光增益介质相同的光频谱范围, 并与 所述第一个法布里 -珀罗标准具有相同的锐度系数和自由光谱范围;其光透射峰值与所述的 第一个法布里-珀罗标准的光透射峰值相差值等于所述可调谐法布里-珀罗滤波器的本征自 由光谱范围。
而且, 所述激光功率锁定器包括一个位于所述激光腔的腔外的光电探测装置, 放置在 被所述第二激光腔反射镜反射并经过所述可调谐声光滤波器衍射的第二次衍射光的零级 光路上。
而且, 所述的激光器驱动控制系统包括: 一个数字信号微处理器 (DSP) 和嵌入式激 光器控制软件、 四个数模转换模块、 一个激光泵浦源、 一个有源光相位调制器驱动源、 一 个射频信号源、一个可调谐法布里-珀罗滤波器的驱动源、一个半导体制冷设备的温控设备、 两个模数转换模块、 一个激光频率和谐振模锁定设备、 一个激光功率锁定设备; 数字信号 微处理器直接驱动控制所述温控设备, 并通过四个数模转换模块分别控制所述激光泵浦 源、 有源光相位调制器驱动源、 射频信号源、 可调谐法布里-珀罗滤波器的驱动源; 通过两 个模数转换模块驱动控制所述激光频率和谐振模锁定设备和所述激光功率锁定设备, 实现 对激光增益介质、有源光相位调制器、 可调谐声光滤波器和可调谐法布里-珀罗滤波器、温 度控制设备、 激光频率和谐振模锁定设备、 激光功率锁定设备的驱动控制功能, 并实现光 频率调谐和锁定以及输出光功率的控制和锁定功能。
本发明的优点和积极效果是:
1、本激光器系统利用液晶对光的相位调制和在通光方向尺寸薄(约 10微米)的特点, 结合传统的法布里 -珀罗 ( Fabry-Perot ) 标准具的技术, 设计了可调谐法布里 -珀罗 ( Fabry-Perot)滤波器, 并结合可调谐声光滤波器, 不仅降低了对可调谐声光滤波器的窄 的滤波带宽到要求, 而且实现在宽频谱范围内的激光频率的快速精密调谐, 保证了大的调 谐光谱范围和窄的激光器输出光谱。 2、 本激光器利用激光腔内反射镜, 不仅实现了激光腔内光束的二次衍射, 克服了由 具有单一声光晶体和单一换能器的可调谐声光滤波器通常所产生的光频率漂移, 使得激光 器的输出光谱更加稳定, 而且, 根据不同的激光器增益介质和应用要求, 采用不同的反射 镜类型与两个激光器的端面反射镜构成不同类型的激光腔型, 同时调节所述激光腔内反射 镜与所述可调谐声光滤波器的距离, 可以调节激光腔的损耗。
3、 本激光器利用可调谐声光滤波器在激光腔内产生的三个零级衍射光束, 在不增加 激光器腔内零部件和不需要在输出光路上插入分光器件的情况下, 实现了激光器光频率和 谐振模以及光功率的监控和锁定。 不仅降低了激光器的成本, 又提高了激光器的性能。
4、本发明设计合理, 可实现在宽频谱范围内光频率调谐精度小于 1GHz和窄频谱带宽 的稳定激光输出, 具有无机械移动部件、 性能稳定可靠、 成本低、 尺寸小、 易于安装及生 产等特点,可满足对于要求尺寸小和极端工作环境下的可靠运行,可广泛应用于光学测试、 光纤通讯、 生物、 医疗器械和光纤传感器网络等领域中。
附图说明
图 1是一种普通可调谐声光滤波器示意图;
图 2是一种基于可调谐声光滤波器的可调谐激光器 100示意图;
图 3-1是声光晶体中一次衍射入射光束、 声波场和衍射光束的波矢关系图; 图 3-2为在声光晶体中二次衍射入射光束、 声波场和衍射光束的波矢关系图; 图 4是一种普通法布里 -珀罗 ( Fabry-Perot) 光标准具的示意图;
图 5是一种包含一个液晶模块的可调谐法布里-珀罗滤波器的示意图;
图 6是液晶模块在外电场作用下光相位和电场的关系曲线示意图;
图 7是普通法布里 -珀罗 ( Fabry-Perot) 光标准具的透射光谱示意图;
图 8是一种可调谐法布里 -珀罗滤波器的透射光谱示意图;
图 9是本发明的结构示意图;
图 10是本发明的激光器驱动控制系统的原理框图;
图 11是用于锁定激光器频率和谐振模的两个法布里 -珀罗标准具的透射光谱。
具体实施方式
以下结合附图对本发明实施例做进一步详述。
图 1是一种现有的可调谐声光滤波器的结构示意图, 该可调谐声光滤波器只有一个换 能器,入射光束 2以布拉格角入射到声光晶体 30,产生零级衍射光束 3 和一级衍射光束 4。
声光滤波器的工作原理是基于一种布拉格衍射的现象。 布拉格衍射涉及了光子 (光能 的量子) 和声子 (声能的量子) 的相互作用过程。 在这个互作用的过程中, 能量和动量都 是守恒的。
动量守恒要求 hKd = hKi + hKs, 其中 hKd是衍射光子的动量, hKi是入射光子的动量, hKS是互作用的声子的动量。 约分去掉 h后得到: Kd = Ki + KS, 这就给出了布拉格衍射最 基本的波矢等式。 它表明了衍射光的波矢是入射光波矢与声波波矢的矢量和, 如图 3-1所 示。
能量守恒要求 h(Dr = h(D + M2, 其中 (Dr是衍射光的角频率, ω是入射光的角频率, Ω 是声波的角频率。 约分去掉 h后得到:
(Dr = ω + Ω。 这表明衍射光子的角频率被声波的角频率轻微改变, 即光线的频率产生 了多普勒频移。
声光可调谐滤波器 (AOTF) 是一种固态的、 可采用电调谐的带通光滤波器。 与传统 的技术相比, AOTF提供了连续、 快速的调节能力和窄的光谱带宽。 声光滤波器有两种类 型: 共线型与非共线型。 其中具有高射频频率的非共线型和非近轴滤波器比较容易实现窄 带滤波。 然而根据上面的公式, ωΓ = ω + Ω, 公式表明光波频率偏移的大小等于声波的频 率。
尽管因为光线频率和声波频率相差很多个数量级, 从而产生的偏移量很小, 但是在一 些激光器系统中还是会引起不稳定的运行。 这个问题的一个解决办法是使用两个 AOTF, 其中第二个 AOTF用来抵消第一个 AOTF所带来的频率偏移。或在同一个声光晶体上使用 两个换能器。 但是这些解决办法都有几个缺点: 1、 增加了系统的体积; 2、 使得光学对准 更为困难; 3、 引起运行的不稳定性; 4、 增加成本, 对大批量生产来说尤为重要。
图 2给出了一种激光器 100的结构。 在该激光器中, 反射镜 45、 50、 和 57组成激光 器的谐振腔。 由激光增益介质 47发出的宽带荧光光束 48经腔内准直透镜 49准直后的光 束 2以布拉格角进入可调谐声光滤波器内的声光晶体 30,第一次衍射光的一级衍射光 4经 腔内反射镜 50反射后以布拉格角再次进入声光晶体 30, 第二次衍射后的一级衍射光 8由 第二激光腔反射镜 57反射回激光腔内, 在激光腔内形成激光振荡和放大。 在这个过程中, 光束 3因其具有最大的能量和经可调谐声光滤波器的二次衍射后波长零漂移而作为激光输 出光束, 其他零级衍射光束 6、 7和 9作为激光腔内光束的泄漏出激光腔外; 光束 6、 7和 9 可用于监控激光腔内的光功率和波长等, 可以避免在激光腔内或输出光路上插入其他分 光器件去实现这样的功能。 激光器 100结构紧凑, 反射镜 45、 50、 和 57可以采用全反镜 组成激光器的谐振腔, 进一步降低激光腔的损耗, 采用了具有单一晶体和单一换能器的声 光可调谐滤波器,可以消除频率的多普勒飘移。入射光束 2以布拉格角入射到声光晶体 30, 产生零级衍射光束 3 和一级衍射光束 4, 一级衍射光束 4经腔内全反射镜 50反射后再次 以布拉格角进入声光晶体 30后产生零级衍射光束 6和一级衍射光束 8。换能器与射频信号 源 10相连接, 该射频信号源 10作为可调谐声光滤波器的驱动源, 提供射频能量并通过改 变射频频率来调节激光谐振腔的振荡波长。
图 3-1和图 3-2显示了入射光 (Ki)、 衍射光 (Kd) 和声波 (KS) 的波矢关系。 正如上 面提到的, Ki ±KS = Kd这个关系永远成立, 使用加号 (+ ) 还是减号 (-) 由入射声波的方 向决定。
在图 3-1中, 光线 2 (κ2)、 光线 4 (κ4) 和声波 40 (KS) 的关系是: K2 + KS = K4。 声 波 KS不仅仅使得衍射光的方向向上偏移, 光线的角频率 ω 也向上偏移了 Q = VS ksl, 其 中 vs是声波的速度。
在图 3-2中, 光线 5 (κ5)、 光线 8 (κ8 ) 和声波 40 (KS) 的关系是: K5 _ KS = K8。 在 这种情况下,声波使得衍射光的方向向下偏移,并且将第二次衍射的光线 8的角频率 ω 也 向下偏移了 VS IKSI。 因为向上和向下的偏移量基本相同, 当光线 8从声光滤波器中射出时, 整体频率偏移被充分的消除了。
在一些具体实施中, 例如需要窄带调节时, 采用的声光晶体是各向异性并有双折射特 性。 其中一种物质为二氧化碲 (Te02), 由于其运行在剪切模式时具有高光学均匀性、 低 光吸收度和耐高光功率能力的特点, 广泛使用于这类应用中。 其他物质例如铌酸锂 ( LiNb03 )、 磷化镓 (GaP)和钼酸铅 (PbMo04 ) 也经常用于各种声光器件中。 影响选择 特定物质的因素有很多, 下面仅列出几种, 如: 声光器件的类型、 高质量晶体是否容易获 得以及应用的类型和需求, 例如衍射效率功率损耗、 入射光与衍射光的分散度和整体器件 的大小等。
图 4给出了一种普通的法布里 -珀罗 ( Fabry-Perot)光标准具 43的示意图。 该法布里 -珀罗光标准具 43 的材料一般在近红外和可见光波段采用象融石英或 BK7这样的光学玻 璃, 假设材料的折射率为 n, 两个通光面 41 和 42都镀高反射膜, 假设反射率为 R, 厚度 为 h, 光以接近零度的入射角入射, 则光标准具 43 的自由光谱范围 FSR1 可以表示为: Δλ^λ2/(2η1 ),或用频率表示: Av=c/(2nh),其中 c 是光速。 透射光的峰值频率可以表示为: v=mc/(2nh),其中 m 是干涉级次, 透射光的频率宽带可以表示为: Δν ( FWHM ) =c(l-R)/(2nhRl/2), 其中 c是光速。
从上述两个公式可以看出, 光标准具 43的自由光谱范围 FSR1 与厚度为 h成反比。 假设材料的折射率为 n=1.5, 要实现 FSRl=100GHz, 厚度 h-1毫米。 FSR1要求越大, 厚 度就越小。 在标准具的材料和厚度确定后, 透射光的频率宽带主要和反射率 R有关, 反射 率越高, 频率宽带或锐度(finesse)越小。 法布里 -珀罗 ( Fabry-Perot)光标准具的透射光 谱的特点是每个透射谱的带宽非常窄, 输出光谱的频率间隔相等并且光频带宽度非常宽, 如图 7所示。
一般情况下, 对于光纤通讯用的激光器, 要求有很窄的输出频率宽带, 也相应地要求 采用高锐度系数的标准具。
一般用作光电器件的液晶材料具有高的电阻率。 因此, 可以被认为是理想的电介质材 料。 由于构成分子的有序的取向和拉伸延长的形态, 液晶具有各向异性的电介质特性和单 轴对称性, 就象一个单轴晶体一样, 其光轴的方向与分子的排列取向一致。 当液晶分子在 外界电场的作用下, 会形成电偶极子。 在电偶极子所形成的力矩作用下, 使得液晶分子的 取向转向电场的方向, 可以通过改变电场的强弱, 改变液晶的光轴的方向。 因此, 可以利 用液晶的这一特性, 制作光相位调制器, 可调谐滤波器, 或其他光电器件, 如光开关和光 强调制器等。 一般用作光电器件的液晶膜层的厚度约为 10 微米。 本发明正是利用这个特 性设计发明了可调谐法布里-珀罗滤波器。
图 5 是一种利用液晶在电场作用下对线偏振光的产生折射率改变而设计的可调谐法 布里-珀罗滤波器。 该可调谐法布里 -珀罗滤波器 200包括两片光学透明材料 16和 26, 以 及在两片光学透明材料的表面 18和 29上镀的反射率为 R的高反射多层电介质膜层, 两反 射膜层形成法布里 -珀罗 (Fabry-Perot)腔; 在透射膜层上分别镀增透膜和设置透明电极膜 层 21和 28, 控制信号 22在两透明电极膜层之间形成驱动电场; 在 21和 28两膜层之间形 成的空腔内设置液晶膜层 24。 由于液晶的厚度很小 (约 10微米), 因此, 可以制作本征自 由光谱范围 (在无外加电场时, 滤波器的自由光谱范围)法布里-珀罗滤波器。 利用电场改 变 Fabry-Perot腔内液晶的有效折射率 n,来调节法布里-珀罗滤波器的透射光的光频率 v和 自由光谱范围 (FSR)。 通常的驱动电场是电压为几伏, 频率为几赫兹到几千赫兹的方波信 号。
如图 6所示, 入射到滤波器 200的光束 15是一束沿 z 方向传播, 偏振轴为 X 方 向的线偏振光, 假设光透明材料的折射率为 n, 两个通光面 18 和 26都镀高反射膜, 假设 反射率为 R, 厚度为 D, 则滤波器 200的
自由光谱范围和透射光频率分别为:
Δλ^λ2/(2ηϋ+Γ),或用频率表示: Av=c/(2nD+r),其中 c 是光速, Γ代表由液晶在外加电场 作用下对入射光所产生的附加光程。透射光的峰值频率可以表示为: V=mC/(2nD+r),其中 m 是干涉级次, 透射光的频率宽带可以表示为: Δν (FWHM) =c(l-R)/((2nD+r)Rl/2), 其中 c 是光速。
图 6显示的是一个液晶在 ΙΚΗζ方波电压的驱动下,对光波长为 1550纳米的光波相位 变化的关系。 最大可实现约 6π的光相位延迟。 根据上述公式, 可调谐法布里 -珀罗滤波器 200对于接近零度入射的线偏振光可以得到约 100GHz的透射光频率的调谐范围, 精度可 达小于 lGHz。 相比较而言, 根据上面的公式, 对自由光谱范围 Δν和透射光的频带宽带的 改变要小的多。
图 8给出了可调谐法布里 -珀罗滤波器的透射光谱示意图。
因此, 可调谐法布里 -珀罗滤波器 200 在外加电场的作用下, 可以实现较大范围的透 射光峰值频率的调谐而基本不改变透射光的频率宽带和自由光谱范围。 这个特性对于将可 调谐法布里 -珀罗滤波器 200在本发明中的应用具有重要意义。
下面对本宽带连续可调谐激光器的结构进行详细说明。
一种宽带连续可调谐激光器, 如图 9所示, 该宽带连续可调谐激光器 300使用了图 1 所示的可调谐声光滤波器和可调谐法布里 -珀罗滤波器 200, 其具体结构为: 包括直接镀在 激光增益介质 47上的第一激光腔反射镜 45、 激光增益介质 47、 腔内准直透镜 49、 有源光 相位调制器 51、 可调谐声光滤波器 100、 腔内反射镜 50、 可调谐法布里 -珀罗滤波器 200、 第二激光腔反射镜 57, 其中, 第一激光腔反射镜 45、 腔内反射镜 50和第二激光腔反射镜 57构成了激光谐振腔。 可调谐法布里 -珀罗滤波器 200设置在可调谐声光滤波器第二次衍 射的一级衍射光束的光轴方向上, 第二激光腔反射镜设置在可调谐法布里-珀罗滤波器后 面。 包括第一激光腔反射镜 45的宽带激光增益介质、 腔内准直透镜、 有源光相位调制器、 可调谐声光滤波器和腔内反射镜安放的位置使得只有经过可调谐声光滤波器二次衍射的 一级衍射光线才能在激光谐振腔内形成激光振荡。
激光腔反射镜通常对不同波长或颜色光的反射率不同, 这里提到的反射率是与激 光器运行的频谱带宽相对应的反射率。 第一激光腔反射镜 45 可以根据不同的情况, 采用 全反镜, 或部分反射镜。 如果激光增益介质是半导体增益介质时, 由于一般都有比较大的 输出分散角, 因此, 腔内准直透镜 49一般是针对激光增益介质是半导体增益介质时使用。 当激光增益介质是气体, 液体或有些固体介质时, 一般不用腔内准直透镜, 而是采用非平 面腔镜以实现腔内光束的合理分布。
腔内准直透镜 49不仅可以将激光增益介质 47发出的光起到准直作用, 同时也是激光 器输出光束的准直透镜。用于光纤通讯中的这类激光器,需要将输出光束 4藕合到光纤中, 准直透镜 49是必不可少的。
在可调谐激光器 300中, 由激光增益介质 47发出的宽带荧光光束 48经腔内准直透镜 49准直后的光束 2透过有源光相位调制器 51, 以布拉格角进入可调谐声光滤波器内的声 光晶体 30, 第一次衍射的一级衍射光 4经腔内反射镜 50反射后以布拉格角再次进入声光 晶体 30, 第二次衍射后的一级衍射光 8透过可调谐法布里 -珀罗滤波器 200 后由第二激光 腔反射镜 57 反射回激光腔内, 在激光腔内形成激光振荡和放大。 在这个过程中, 零级衍 射光束 3、 6、 7和 9作为激光腔内光束的零级光束溢出激光腔外; 光束 3在激光振荡和放 大过程中因其具有最大的能量和经可调谐声光滤波器的二次衍射后波长零漂移而作为激 光输出光束。 光束 6、 7和 9 可用于监控激光腔内的光功率和波长等。 下面将会详细说明。
正如前面分析的, 由于第一次衍射的一级衍射光的频率偏移和第二次衍射的一级衍射 光所产生的光频率偏移正好相反, 因此, 可调谐声光滤波器在可调谐激光器 300中的结构 中对激光器输出所造成的光频率偏移为零。 又由于经可调谐声光滤波器的两次衍射, 在激 光腔内形成了比一次衍射带宽更窄的激光振荡。
根据增益介质的情况, 采用不同类型的腔内反射镜 50和第二激光腔反射镜 57可 以形成不同的激光腔, 还可以补偿由于声光晶体衍射造成的腔内光束的色散和发散, 降低 激光腔的损耗,或采用反射光栅或其他色散器件进一步压縮腔内光束的光谱等优点。同时, 调节腔内反射镜 50和声光晶体 30之间的距离 L (见图 2), 也可以调节激光腔的损耗。
可调谐激光器 300输出的光频率是通过控制有源光相位调制器 51,驱动可调谐声光滤 波器射频信号源 10和可调谐法布里 -珀罗滤波器 200来实现调谐的。 首先, 改变可调谐声 光滤波器的射频信号源 10 的射频频率, 可选择激光腔内的衍射光的频率。 根据激光腔内 不同的光频率, 有源光相位调制器 51 通过调节腔内光束的相位使得某一个特定频率的光 在激光腔内产生激光振荡和放大。在可调谐法布里 -珀罗滤波器 200无外加电场作用时, 这 时的滤波器 200相当于一个法布里-珀罗标准具,可调谐激光器 300输出的光频率受滤波器 200的本征透射光谱的限制, 即可调谐激光器 300的输出只能调谐在可调谐法布里-珀罗滤 波器 200 的本征透射光谱的其中一个透射光谱。 通过调节驱动可调谐法布里 -珀罗滤波器 200的外加电场, 可以精密调谐激光器 300的输出光谱。 在精密调谐过程中, 也需要微调 有源光相位调制器 51和射频信号源 10的射频信号。 由于有源光相位调制器 51, 可调谐声 光滤波器和可调谐法布里 -珀罗滤波器 200均具有很宽的光频谱范围, 因此, 可调谐激光器 300就能够实现在宽频谱范围内的精密连续可调谐。
例如: 对于滤波器 200 的本征透射光谱是满足国际电讯联盟 (ITU) 标准的 100GHz 的 DWDM系统, 可调谐激光器 300通过调节有源光相位调制器 51和可调谐声光滤波器 100, 输出光谱可以满足 ITU 100GHz 的光频率要求 (ITU Grid)。 如前面所述, 在可调谐 法布里 -珀罗滤波器 200有外加电场作用时,可以实现 100GHz的透射光的频谱的精密频率 调谐, 因此, 可实现在两个 lOOGHz I U grid光频率的之间的精密和连续调谐。 一般可以 做到间隔小于 1GHz的光频率精密调谐。 目前, 在光纤通讯中, 常用的 C频率带(约 1530 纳米 -1570纳米)或 L频率带(约 1570纳米 -1610纳米) 的光频谱宽带约为 40纳米, 可调 谐激光器 300完全可以实现在 C频率带和 /或 L频率带范围内的精密调谐。
同时, 可调谐法布里 -珀罗滤波器 200也决定了激光器输出光的频谱宽度。采用高锐度 系数的法布里-珀罗滤波器能起到压縮输出光束的频谱带宽和提高边模抑制比。由于调谐过 程对法布里 -珀罗滤波器的透射光谱的带宽影响可以忽略, 因此, 可调谐激光器 300在调谐 过程中, 输出光谱的带宽可以做到基本一致。
一般情况下, 对声光可调谐滤波器的滤波带宽的要求, 应使得可调谐激光器 300的腔 内的谐振模的本征频谱带宽的半宽度(即在腔内未设置可调谐法布里 -珀罗滤波器 200时的 谐振模的频谱的半宽度)小于可调谐法布里 -珀罗滤波器 200的本征自由光谱范围, 以保证 可调谐激光器 300的腔内实现单模运行。由于可调谐法布里 -珀罗滤波器 200在调谐过程中, 基本不改变自由光谱范围, 使得在激光器 300的调谐过程中, 能够继续维持单模运行。 但 由于器件老化等原因, 激光器 300可能会出现谐振模的跳变, 即激光器从一个谐振模跳到 相邻模上, 或其他离得更远的模上运行, 频率和光功率等主要参量也有可能出现漂移。 频 率漂移指的是在激光器工作在不发生跳模时发生的频率漂移现象, 一般地, 频率漂移中的 频率变化比较小。 因此, 为了实现激光器的稳定工作, 有必要对上述三个参量: 谐振模, 频率和光功率进行实时监控和锁定。
激光谐振模锁定的实现
激光器 300的谐振模和频率实时监控和锁定是通过在泄漏出激光腔外的零级衍射光 6 和 9 (图 9) 上设置的具有低锐度系数 (low Finesse) 的法布里 -珀罗标准具 62和 64, 以 及位于上述标准具之后的光电探测设备 60和 66 以及相关的锁定设备实现的。 具有低锐 度系数(low Finesse)的法布里 -珀罗标准具的透射频谱近似于一个正弦曲线。 图 10和图 11分别显示的是具有相同锐度系数的标准具 62和 64的透射光谱曲线. 假设可调谐法布里- 珀罗滤波器 200的本征自由光谱范围为 FSR1 则法布里 -珀罗标准具 62的自由光谱范围为 四倍的 FSR1,而且, 透射光谱的第一个频率 Λ/1与可调谐法布里 -珀罗滤波器 200的第一个 透射频率 vl相同, 法布里 -珀罗标准具 64的自由光谱范围也为四倍的 FSR1,而且, 透射光 谱的第一个频率与可调谐法布里 -珀罗滤波器 200Λ/2相同 (或者说, 法布里 -珀罗标准具 62 和 64的透射峰值相差四分之一的 FSR1 )。 这样, 对于不同的频率的光在光电探测设备 60 和 66上得到的光功率强度不同。 通过归一化, 假设最大峰值透射光强为 1, 最小透射光强 为零, 中间值为 0.5, 则各频率在光电探测设备 60和 66的光强值列于下面的表格: 光强(光电设备 60) 光强 (光电设备 66) vl 1 0.5
v2 0.5 1
v3 0 0.5
v4 0.5 0
v5 1 0.5
v6 0.5 1
v7 0 0.5
v8 0.5 0
v9 1 0.5
vlO 0.5 1 由于透射光谱具有周期性特点, 表中仅列了前十个频率的光强值。 从上述表中可以看 出, 通过对光电设备 60和光电设备 66的两个光强值的比较, 可以区别 vl同 v2、 v3、 v4、 ν6、 νΊ、 v8; v2同 vl、 v3、 v4、 v5、 v7、 v8、 v9, 可依次类推。 vl、 v5 、 v9禾卩 v2、 v6 、 vlO呈现周期性重复, 可依次类推。 在激光器 300的校准过程中, 将每个频率对应的光强 值在数字信号微处理器中形成一个查找表格 (Lookup Table), 就可以判断激光器 300是 否发生跳模, 通过激光器驱动控制系统可将谐振模锁定。需要指出的是, 工作在 vl的激光 器如果发生跳模至 v5 或 v9, 或工作在 Λ/2的激光器如果发生跳模至 Λ/6或 vlO, 则系统将 无法判断, 其他模也可依次类推。 因此, 本发明只能对跳变到非周期性模上的情况, 才能 作出判断。 一般情况下, 激光器的跳模会跳变到邻近模上, 则, 本发明只能对该模的上 和 /或下各三个模的情况作出判断。
在模式锁定过程中, 不仅需要判断是否发生跳模, 还需要能够判断
跳模的方向, 才能有依据通过改变相应的激光器驱动参数, 如改变驱动可调谐声光调 制器的射频频率, 把谐振模调整到原来的谐振模上。再以表中的 v5为例, 由于系统不能区 分 Λ/3和 v7, 而只能判断 Λ 和 v6。 因此, 本发明只能对发生模跳变到最邻近的两个模的情 况实施锁定。
如要扩大激光模的判断范围,需要增加探测设备和改变法布里-珀罗标准具的自由光谱 范围。 例如, 再增加一路光电探测器和一个法布里-珀罗标准具, 将三个布里 -珀罗标准具 的自由光谱范围扩大到所述可调谐法布里-珀罗本征自由光谱范围 FSR1的八倍并使其峰值 透射频率相互错位二倍的 FSR1 , 可以实现对模跳变到最邻近的四个模的情况实施锁定。
激光频率锁定的实现 即使激光器的谐振模不发生跳变, 由于器件老化等原因, 也可能使激光器频率产生漂 移。 一般地, 频率漂移指的是频率的变化不超过两个谐振模频率间隔的一半 (即等于
0.5(v2-vl) o 参考图 11, 当频率发生漂移时, 因为透过布里 -珀罗标准具 62和 64的光强就 会发生变化, 因此, 在光电设备 60和 66上探测到的光强也会发生变化。利用光强的变化, 可以检测光频率的变化并通过激光器 300的驱动控制系统来进行对频率进行锁定, 包括改 变腔的温度、 相位和射频信号频率等。 从图 11可以看出, 激光模 vl、 v3、 v5、 W等奇数 模位于图 11下方的透过率曲线的中央区域, 而 v2、 v4、 V6、 V8 等偶数模位于图 11上方 的透过率曲线上升沿或下降沿的中央区域. 因此, 在该区域光强随光频率的变化具有最好 的线性度。 可以通过探测光电设备 60 的光功率的变化对偶数模进行监控, 通过光电设备 66的光功率的变化对奇数模进行监控,并通过激光器 300的驱动控制电路系统的闭环控制 回路进行激光频率锁定。
激光功率锁定的实现
参考图 9,光电探测设备 68用于直接监控激光器 300的功率变化并通过激光器的驱动 控制电路系统的闭环控制回路并通过改变激光器的泵浦功率等参数进行激光输出功率的 锁定。
上述可调谐激光器 300的驱动控制电路系统如图 10所示。 该驱动控制电路系统包括: 带有嵌入式软件程序的数字信号微处理器 (DSP ) 120 、 四个数模转换 (D/A) 设备 110、 112、 113和 115、 分别用于驱动控制激光增益介质 47的激光泵浦源 109、 有源光相位调制 器的相位调制器驱动源 111、可调谐声光滤波器 100的射频信号源 10、可调谐法布里 -珀罗 滤波器的法布里 -珀罗滤波器驱动源 114、 两个模数转换(A/D )设备 117和 119, 分别用于 驱动控制激光频率和谐振模锁定设备 116和激光功率锁定设备 118。 同时数字信号微处器 120直接驱动控制温控设备 70, 也可以接收外部指令来并通过四个数模转换模块分别控制 所述激光泵浦源、有源光相位调制器驱动源、射频信号源、 可调谐法布里-珀罗滤波器的驱 动源; 通过两个模数转换模块驱动控制所述激光频率和谐振模锁定设备和所述激光功率锁 定设备, 实现对激光增益介质、 有源光相位调制器、 可调谐声光滤波器和可调谐法布里- 珀罗滤波器、 温度控制设备、 激光频率和谐振模锁定设备、 激光功率锁定设备的驱动控制 功能, 并实现光频率调谐和锁定以及输出光功率的控制和锁定功能。
需要强调的是, 上述说明仅起演示和描述的作用, 并不是一个详细无遗漏的说明, 也 没有意图将本发明限制在所描述的具体形式上。 经过上面的描述, 对本发明的许多改动和 变化都可能出现。 所选择的具体实施仅仅是为了更好的解释本发明的原理和实际中的应 用。 这个说明能够使熟悉此领域的人可以更好的利用本发明, 根据实际需要设计不同的具 体实施和进行相应的改动

Claims

权利要求书
1、 一种宽带连续可调谐激光器, 包括依次安装在激光腔内的第一激光腔反射镜、 激 光增益介质、 腔内准直透镜、 有源光相位调制器、 可调谐声光滤波器, 其特征在于: 还包 括腔内反射镜、 可调谐法布里-珀罗滤波器、第二激光腔反射镜及激光器驱动控制系统; 所 述腔内准直透镜用于将所述激光增益介质发出的光准直, 并以布拉格角进入所述可调谐声 光滤波器; 所述腔内反射镜放置的位置使得由所述可调谐声光滤波器第一次衍射的一级衍 射光被再次反射到所述可调谐声光滤波器中而得到第二次衍射, 并能消除由第一次衍射造 成的光频率多普勒漂移; 在第二次衍射的一级衍射光的光轴方向上设置所述可调谐法布里 -珀罗滤波器, 在所述可调谐法布里-珀罗滤波器后设置所述第二激光腔反射镜; 由所述的 第一激光腔反射镜、 所述的腔内反射镜和所述的第二激光腔反射镜构成了激光器谐振腔; 所述的第一激光腔反射镜、 宽带激光增益介质、 腔内准直透镜、 有源光相位调制器、 可调 谐声光滤波器、可调谐法布里 -珀罗滤波器和腔内反射镜安放的位置使得只有经过所述可调 谐声光滤波器的第一次和第二次衍射的一级衍射光的光线才能在所述激光谐振腔内形成 激光振荡; 所述宽带连续可调谐激光器的输出光束和第一次衍射的零级光重合。
2、 根据权利要求 1 所述的一种宽带连续可调谐激光器, 其特征在于: 所述第一激光 腔反射镜、 腔内反射镜和第二激光腔反射镜均为下列几种反射镜之一: 平面镜, 凹面镜和 凸面镜, 具有部分或 100%的反射率并与所述激光增益介质具有相同光谱范围; 所述第一 激光腔反射镜或者是直接镀在激光增益介质一个端面上的多层介质膜; 所述腔内反射镜或 者是一种反射型光栅或其他具有光色散的反射镜。
3、 根据权利要求 1 所述的一种宽带连续可调谐激光器, 其特征在于: 所述激光增益 介质是一种宽带激光增益介质。
4、 根据权利要求 1 所述的一种宽带连续可调谐激光器, 其特征在于: 所述有源光相 位调制器可以是下列几种类型之一: 电光相位调制器、 声光相位调制器、 磁光相位调制器 或上述几种相位调制器的某种组合。
5、 根据权利要求 1 所述的一种宽带连续可调谐激光器, 其特征在于: 所述可调谐声 光滤波器包括一块声光晶体和设置在所述声光晶体上的一个电声换能器。
6、 根据权利要求 1 所述的一种宽带连续可调谐激光器, 其特征在于: 所述可调谐法 布里-珀罗滤波器由第一反射镜、液晶模块和第二反射镜依次连接构成; 在没有外加电场时 的本征自由光谱范围介于 10GHz和 400GHz之间, 由所述的第一反射镜的第一个通光面和 所述的第二反射镜的第二个通光面构成法布里-珀罗腔。
7、 根据权利要求 6 所述的一种宽带连续可调谐激光器, 其特征在于: 所述的第一反 射镜的第一个通光面设置高反射率多层介质膜; 所述的第一反射镜的第二个通光面依次设 置光学增透膜、 透明电极和电极隔离膜;
8、 根据权利要求 6 所述的一种宽带连续可调谐激光器, 其特征在于: 所述液晶模块 的材料是一种向列相型液晶材料,厚度为几微米到十几微米,在外加电场的驱动下对某一方 向的线偏振光具有至少 2 II的光相位延迟, 并具有与激光增益介质相同的光谱范围。
9、 根据权利要求 6 所述的一种宽带连续可调谐激光器, 其特征在于: 所述的第二反 射镜的第一个通光面的第一层设置非导电材料薄膜, 覆盖除通光孔径以外的部分, 厚度为 几微米到十几微米; 所述的第二反射镜的第一个通光面的第二层设置光学增透膜; 所述的 第二反射镜的第一个通光面的第三层设置透明电极; 所述的第二反射镜的第二个通光面设 置高反射率多层介质膜并与所述的第一反射镜的第一个通光面具有相同的反射率。
10、 根据权利要求 1所述的一种宽带连续可调谐激光器, 其特征在于: 所述激光频率 和谐振模锁定器包括: 第一个法布里-珀罗标准具, 放置在所述第二次衍射的零级光的光路 上并位于所述激光腔的腔外; 第一个光电探测装置, 放置在所述第一个法布里 -珀罗标准具 后, 用于检测从所述第一个法布里-珀罗标准具透过的光的功率; 第二个法布里-珀罗标准 具, 放置在被所述第二激光腔反射镜反射后并经所述可调谐声光滤波器衍射的零级光的光 路上, 位于所述激光腔的腔外; 第二个光电探测装置, 放置在所述第二个法布里-珀罗标准 具后, 用于检测从所述第二个法布里-珀罗标准具透过的光的功率。
11、根据权利要求 10所述的一种宽带连续可调谐激光器, 其特征在于: 所述第一个法 布里-珀罗标准具有与所述激光增益介质相同的光频谱范围,其锐度系数使得透射光强最大 值和最小值的差大于 3dB;其自由光谱范围是所述可调谐法布里 -珀罗滤波器的本征自由光 谱范围的四倍;其光透射峰值的某一频率与所述可调谐法布里-珀罗滤波器的光透射峰值的 某一频率相同。
12、 根据权利要求 10 所述的一种宽带连续可调谐激光器, 其特征在于: 所述第二个 法布里-珀罗标准具有与所述激光增益介质相同的光频谱范围, 并与所述第一个法布里-珀 罗标准具有相同的锐度系数和自由光谱范围;其光透射峰值与所述的第一个法布里-珀罗标 准的光透射峰值相差值等于所述可调谐法布里-珀罗滤波器的本征自由光谱范围。
13、 根据权利要求 1所述的一种宽带连续可调谐激光器, 其特征在于: 所述激光功率 锁定器包括一个位于所述激光腔的腔外的光电探测装置, 放置在被所述第二激光腔反射镜 反射并经过所述可调谐声光滤波器衍射的第二次衍射光的零级光路上。
14、 根据权利要求 1至 13任一项所述的一种宽带连续可调谐激光器, 其特征在于: 所述的激光器驱动控制系统包括: 一个数字信号微处理器和嵌入式激光器控制软件、 四个 数模转换模块、 一个激光泵浦源、 一个有源光相位调制器驱动源、 一个射频信号源、 一个 可调谐法布里-珀罗滤波器的驱动源、一个半导体制冷设备的温控设备、两个模数转换模块、 一个激光频率和谐振模锁定设备、 一个激光功率锁定设备; 数字信号微处理器直接驱动控 制所述温控设备, 并通过四个数模转换模块分别控制所述激光泵浦源、 有源光相位调制器 驱动源、射频信号源、 可调谐法布里-珀罗滤波器的驱动源; 通过两个模数转换模块驱动控 制所述激光频率和谐振模锁定设备和所述激光功率锁定设备, 实现对激光增益介质、 有源 光相位调制器、 可调谐声光滤波器和可调谐法布里-珀罗滤波器、温度控制设备、激光频率 和谐振模锁定设备、 激光功率锁定设备的驱动控制功能, 并实现光频率调谐和锁定以及输 出光功率的控制和锁定功能。
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