PASSIVELY MODE-LOCKED SEMICONDUCTOR LASER

Abstract: A miniature ultra-short light pulse source is based on a passively mode-locked semiconductor laser, including a multisegment laser diode (LD) with controllable saturation in absorbing sections(s) and anti-reflection coating on one of its facets, which is used as an active element. A single-mode optical fiber with a Bragg grating (FBG) written into its core is used as an external cavity. Depending on FBG reflectivity spectrum, the device emits a regular train of transform-limited ps-length pulses or linearly chirped ps-length pulses suitable for a time-compression technique.

Description: WO 00/55950 PCTtUSOO/06928 PASSIVELY MODE-LOCKED SEMICONDUCTOR LASER TECHNICAL FIELD This invention relates generally to semiconductor lasers and particularly to external cavity laser diodes integrated with saturable absorbers. BACKGROUND OF THE INVENTION Semiconductor lasers are often used as ultra-short light pulse generators. In laser diodes with an external cavity the mode-locking regime — passive, active or hybrid — usually makes it possible to obtain the shortest output pulses available, with picosecond or sub-picosecond duration. The advantage of passive mode locking is that this method does not require microwave equipment. In this case ultra-short light pulses are formed in self-sustained process due to the nonlinear properties of a saturable absorber. This element can be located separately in the same external cavity or can be integrated with laser diode. For example, J.P. van der Ziel et al. first obtained laser diode light pulses shorter than 1 ps (1 picosecond) using saturable absorbers created by proton implantation. Multi-section or multi-segment laser diodes, invented in 1964, are still under research and improvement due to their many functions. In these devices the parameters of the saturable absorber(s), realized as electrically isolated section(s) of laser diode, can be controlled by external bias voltage or leakage current. Under mode-locking conditions it is possible to optimize generated pulse output parameters. Since the discovery of photosensitivity in optical fibers and the development of UV writing of fiber Bragg gratings, these optical elements have found many applications, mainly in optical communication systems and optical sensors. The parameters of fiber Bragg gratings, such as resonant wavelength, reflectivity, spectral bandwidth, length, chirp rate and other parameters, are widely variable. The fiber pigtail with Bragg grating and some coupling element at its facet is often used as a miniature external cavity for laser diode. For example, P.A.MORTON et al. used such a design for creating a hybrid soliton pulse source based on an actively mode-locked laser diode. This source emitted nearly transform-limited light pulses of 20 ps (pico second) duration with a repetition rate tunable near 2.5 GHz, due to the small chirp of the fiber Bragg grating. In principal, fiber Bragg gratings with high enough chirp rate can be used as external time compressors and intra-cavity chirp correctors for sub-picosecond laser diode creation. An object of the present invention is to provide a small-size DC-pumped source of high-power ps and fs (femtosecond) light pulses. This device combines the advantages of a multi-segment laser diode as an active element, fiber Bragg grating as external cavity, and passive mode-locking regime. One obvious disadvantage of this light source is the high jitter peculiar to passively mode-locked lasers. But for several applications, such as 2-photon optical data recording, 2-photon microscopy, and high-speed photodetector metrology the light pulse generator jitter is not a critical parameter. The replacement of solid-state and dye lasers by semiconductor lasers in those applications is highly desirable due to their well- known advantages, such as: high efficiency, small size, high reliability, and potentially low cost. SUMMARY OF THE INVENTION We have found that a passively mode-locked semiconductor laser with a simply designed external cavity can generate a regular train of pure picosecond pulses ("monopulses"). Its external cavity is based on a fiber Bragg grating (FBG) with a multi- segment laser diode (LD) as an active element. The LD nonlinear properties can be controlled by a tunable leakage resistor or bias voltage applied to absorbing section(s). Following an experimental study of multi-segment LD's based on (GaAl) as bulk double heterostructure (DH), separate confinement heterostructure (SC DH) and single quantum well heterostructure (SQW SC DH), we have chosen DH-based device with mostly effective saturable absorbers due to the higher optical confinement factor. The LD's ridge waveguide dimensions are very useful for single spatial mode generation and effective coupling with a single-mode fiber. The DH mesa-structure width W should be equal to 3.0-4.0 μm (micrometers) while its height should equal 1.55-1.65 μ . To ensure high enough electrical isolation between sections without optical losses increasing, the depth of gap separating the sections should be equal to 0.6-0.9 μm, which corresponds to 1-3 kilohm isolation resistance. Anti-reflection-coated facet residual reflectivity should be less than 0.5%. At higher reflectivity the excitation of satellite pulses with a repetition period corresponding to the LD round-trip time (10-30 ps) becomes possible. The lower the insertion losses of coupling element, the lower the threshold current is and the higher the output power is. For coupling efficiency of more than 30%, self-sustained mode locking with a sufficiently wide pumping current range (10-30 mA above threshold) can be obtained. FBG reflectivity should be high enough - more than 10-20% at spectral maximum - to obtain stable mode locking. The output pulse repetition rate depends on the external cavity length L. Stable mode locking is obtainable for cavity lengths between 5 cm and 50 cm, which corresponds to a repetition rate of 0.2- 2.0 GHz. Output light pulse spectral parameters are governed, in the first term, by the FBG reflectivity spectrum. An FBG with narrow-band reflectivity (FWHM=0.1-0.2 nm (nanometers)) should be used to obtain transform-limited ps pulses. On the contrary, to obtain linearly chirped output pulses one should use an FBG with wide-band reflectivity (FWHM>2 nm) together with corresponding tuning of LD operation conditions - pumping current of gain sections and bias voltage (leakage resistance) of absorbing section(s). To avoid spreading of the pulses, which are reflected in FBG with uniform spatial period, the length of such FBG should be short, preferably less then 0.3-0.5 mm. Chirped FBG's can be used as external cavity mirrors for intra-cavity chirp correction and as external time compressors of chirped output light pulses. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of passively mode- locked semiconductor laser of this invention; FIG. 2 is a schematic representation of active element - multi-segment LD of this invention; FIG. 3 is streak-camera chronograms with high (a) and low (b) time resolution of output pulses of laser of this invention; FIG. 4 is time-resolved spectrograms of transform-limited (a) and linearly chirped (b) ps light pulses of laser of this invention; FIG. 5 is a schematic representation of laser of this invention with an external fiber time compressor; FIG. 6 is a schematic representation of laser of this invention with chirped semitransparent fiber Bragg grating; and FIG. 7 is a schematic representation of laser of this invention with chirped high- reflection fiber Bragg grating and semitransparent coating on a front facet of active element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 schematically shows a semiconductor laser with an external fiber cavity, which is the subject of this invention. There are an active element, a multi-segment edge-emitting laser diode 1; a single-mode optical fiber 2, which acts as an external cavity; and a coupling element 3. Fig.l shows one embodiment of a laser diode, a three-section diode which includes an active channel 4 which is divided into gain sections coupled to injecting contact 5 and a section, which acts as a saturable absorber, coupled to contact 6, preferably used as a connection to a leakage resistance or bias voltage. The sections are divided by etched grooves which electrically isolate them from one another. The emitting facet of the laser diode, which is coupled with the external cavity, is coated with anti-reflection coating 7. The opposite facet of laser diode is coated with high-reflection or semitransparent coating 8. The fiber Bragg grating 9 has a uniform, or linearly chirped, spatial period of effective refraction index, as a function of the coordinate along the fiber axis, written into the optical fiber core. This grating acts as a mirror of the external cavity. The repetition rate of pulses generated under passive mode locking depends on the external cavity length L. The spectral position of the grating reflection maximum should correspond to the laser wavelength. The laser linewidth depends upon the grating reflection bandwidth. A grating with a sufficiently large reflection bandwidth (more than «2 nm) and a linear chirp of about 10 nrn/cm can correct light pulse chirp appearing during its spreading along the laser diode active channel. A coupling unit 3 is needed to provide optical coupling between the laser diode active channel and the fiber core. The preferred method is the simplest way of coupling - a microlens created on the laser diode-oriented fiber facet. A micro-objective located between the laser diode's and fiber's facets, or butt coupling, can also be used, although the last method requires a complication of the active element structure. Fig.2 illustrates the active element structure. For better understanding, a two-section laser diode is shown. The illustrated diode is based on a bulk double heterostructure. (GaAl)As heterostructures with a 820-870 nm radiation wavelength were used in the research. The heterostructure was grown on the GaAs N-type substrate 10 and includes an N- emitter 11 with Al 0.35 Gao 65 As composition, an undoped active layer with an Al x Ga, .x As (0<x<0.05) composition about 0.1 μm thick, a P-emitter 12 with an Alo^Gao^ j As composition about 1.5 μm thick, and a contact layer 13 consisting of P + -GaAs with a preferred thickness of 0.3-0.5 μm. The laser ridge-waveguide active channel was created by ion etching through a photolithographic mask. The width W and height of this mesastructure 21 are critical in providing laser radiation of the lowest spatial mode, which is necessary for effective coupling to the single-mode optical fiber. Optimal values of these parameters for the studied heterostructure were 3.0-4.0 μm and 1.55-1.65 μm respectively. The same method was used for etching the grooves 19, dividing gain section 18 and absorbing section 17. An optimal groove depth, providing effective electric isolation resistance between sections of 1-3 kilohm without introducing additional optical losses into the active waveguide, is about 0.6-0.9 μm. The wafer's p-side was covered by a dielectric film 16 with windows, for gain and absorbing sections contacts, opened by the photolithographic method. Ohmic contacts 15 and 17 were deposited on the wafer's n- and p-sides. The photolithographic method was also used for dividing contacts on the p-side. The wafer was afterwards cleaved into 0.5-1.0 mm-wide bars, defining the length of the laser diode cavities. The bars' facets were coated with high-reflection and anti-reflection dielectric films. The bars were cleaved into separate active elements that were soldered p-side up onto copper heat sinks. A thermocompression method was used to connect thin gold wire conductors to the contact areas of the gain and absorbing sections. Gain sections were connected to a DC source of positive polarity. Absorbing sections were grounded through a tunable leakage resistance of 0-100 Ohm or switched to a 0-5 N DC source of negative polarity. During research the optical fiber pigtail with the Bragg grating, acting as an external cavity, was fixed a on 3-D piezo-positioner, providing a positioning accuracy of 0.1 μm on each axis. The pigtailed light-emitting modules assembly technology allows the device, which is the subject of the present invention, to be produced in monolithic packages. Fig.3 shows typical emission chronograms for an embodiment of the laser with a three-section active element of 0.55 mm length and an external cavity of 10 cm length, recorded by streak-camera. The time axis for fast sweep-time was calibrated with 8.2 ps time delay (see Fig.3a). Typical values of output pulses peak power were 2-5 W. The output radiation spectrum depends on the reflectivity bandwidth of the FBG used. The use of narrow-band gratings with a reflection contour having a spectral half- width (spectral full width at half maximum (FWHM)) less than 0.3 nm) makes it possible to obtain nearly transform-limited output pulses. The time-bandwidth product in this case is about 0.4- 0.5. The use of wide-band gratings with a spectrum reflectivity half- width contour over 2 nm causes the generation of output pulses of the same duration, but far from transform-limited ones. Their spectrum bandwidth can reach several nm. Under specific operating conditions the generated pulses are practically linearly chirped. Changing the pumping current or operating conditions of the absorber can control the chirp rate. These trends are illustrated in Fig.4, showing time-resolved spectrograms of the emission of lasers with identical active elements, but different FBG'S in the external cavity (FWHM=0.25 nm at (a), FWHM=2.2 nm at (b)). The above concerns fiber Bragg gratings with uniform spatial period. Gratings with a variable spatial period along the fiber axis in accordance with a certain order (in the simplest case a linear order), the so-called chirped FBG'S, can be used for time compression of output pulses of the laser of the present invention. Figs. 5-7 show variants of such devices, which permit receiving sub-picosecond light pulses. In Fig. 5 a linearly chirped FBG is used as an external time compressor. Output pulses of a passively mode-locked laser 22 with linear chirp arrive at the 3-port fiber circulator 23 input. An FBG 24 with a compensatory linear chirp and 100% reflection is connected to the next circulator port. As a result, the duration of the reflected pulses arriving at the circulator output is compressed. The time-resolved spectrograms of light pulses are shown schematically in circles. In Figs. 6 and 7 chirped FBG'S are used as external cavities. In addition to providing effective optical feedback they also compensate the chirp developed by light pulses during their spreading along the laser diode active channel. The first design utilizes a semitransparent grating 25, which adds a "forerunning" chirp, compensated during the round- trip of the pulse in the laser diode. The design utilizes a chirped grating 27 with a 100% reflection. In this case the radiation is emitted through the external facet of the laser diode with a semitransparent coating 25. The time-resolved spectrograms of light pulses are shown schematically in circles. As stated above, the inventors have manufactured the device with laser wavelength in the range of 820-870 nm, utilizing multi-segment laser diodes based on (GaAl)As double heterostructures. Lasers within the scope of the present invention can also be produced using other types of semiconductor heterostructures. This would require manufacturing multi- segment laser diodes with sufficiently effective saturable absorbers, allowing effective coupling with optical fibers, as well as use of single-mode fibers and FBG'S for the corresponding spectral range. The lasers of the present invention can effectively act as master oscillators for powerful ultra-short light pulse sources, utilizing power amplifiers (MOP A) based on traveling-wave semiconductor optical amplifiers.

Claims: WE CLAIM: 1. A semiconductor laser device comprising an active element, an external cavity, and a coupling element coupling the active element and the external cavity; wherein the active element comprises a multi-segment laser diode including a gain section and an absorbing section; and wherein the external cavity comprises a single-mode optical fiber including a mirror, the mirror comprising a fiber Bragg grating. 2. The laser device as recited in claim 1, wherein the laser diode is an edge-emitting laser diode comprising a double heterostructure. 3. The laser device according to claim 1, wherein the active device includes a ridge- waveguide active channel. 4. The laser device according to claim 3, wherein the active channel includes a width between approximately 2.0 μm and approximately 4.0 μm. 5. The laser device according to claim 1, wherein the active device includes an isolation resistance coupled between the gain section and the absorbing section, the isolation resistance being between approximately 1.0 kilohm and approximately 3.0 kilohm. 6. The laser device according to claim 1 , wherein the active device includes an anti- reflection coating on a facet thereof coupled with the external cayity. 7. The laser device according to claim 6, wherein the anti-reflection coating includes a residual reflectivity less then approximately 0.5%. 8. The laser device according to claim 1, wherein the coupling element includes a coupling efficiency of more than approximately 25%. 9. The laser device according to claim 1, wherein the fiber Bragg grating includes a uniform spatial period. 10. The laser device according to claim 9, wherein the fiber Bragg grating includes a length less than approximately 0.5 mm. 11. The laser device according to claim 9, wherein the fiber Bragg grating includes a maximum spectral reflectivity between approximately 10% and approximately 90%. 12. The laser device according to claim 9, wherein the fiber Bragg grating includes a reflectivity spectrum FWHM between approximately 0.1 nm and approximately 0.5 nm. 13. The laser device according to claim 9, wherein the fiber Bragg grating includes a reflectivity spectrum FWHM over approximately 2.0 nm. 14. The laser device according to claim 1, wherein the fiber Bragg grating includes a linearly chirped spatial period. 15. The laser device according to claim 14, wherein the linearly chiφed spatial period comprises a linear chhprate over 5.0 nrn/cm. 16. The laser device according to claim 14, wherein the fiber Bragg grating includes a reflectivity spectrum FWHM over approximately 2.0 nm.

Inventors: MAGNITSKII S A, SHRAMENKO M V, YAKUBOVICH S D, MAGNITSKII, S. A., SHRAMENKO, M. V., YAKUBOVICH, S. D.
Applicants: FEMTOLINE INC, FEMTOLINE INC.
Application year: 2000
Main classification: H01S5
IPC codes: H01S5