CN105655870B - It is a kind of based on prism beam-expanded tunable grating external-cavity semiconductor laser - Google Patents

Abstract

The invention provides a tunable grating external cavity semiconductor laser based on prism beam expansion. The technical solution is: including a laser diode, a collimating lens, and a grating. It is characterized in that it also includes a prism beam expander system composed of one or more triangular prisms, so that the laser light emitted by the laser diode passes through the collimating lens and before passing through the grating. Prism beam expander system for dispersion amplification. The incident light angle of each prism is the Brewster's angle corresponding to the wavelength of the laser diode. The outgoing light of each triangular prism is evenly perpendicular to the outgoing edge of the triangular prism. After adding the prism beam expander system in the present invention, the line width of the laser is further narrowed.

Description

A tunable grating external cavity semiconductor laser based on prism beam expansion

technical field

The invention relates to semiconductor laser technology, in particular to a tunable grating external cavity semiconductor laser based on prism beam expansion.

Background technique

Semiconductor lasers have been widely used in many fields due to their advantages such as direct electro-optic conversion, small size, and long life. However, their linewidth is usually relatively large. Considering that the linewidth of a laser is directly proportional to the phase noise, narrow linewidth tunable lasers become indispensable in order to guarantee the performance of their related systems. For semiconductor lasers, stimulated radiation is dominant, but spontaneous emission is inevitable, which will cause frequency noise and broaden the laser spectrum. Narrow linewidth tunable lasers are the key optoelectronic devices for the new generation of dense wavelength division multiplexing systems and photon exchange in all-optical networks. At present, continuous or quasi-continuous tuning in a wide wavelength range has been realized, and corresponding products have been put on the market.

Tunable external cavity semiconductor lasers have the advantages of narrow linewidth, large tuning range, high output power, better single longitudinal mode characteristics and stability. In order to make the semiconductor laser output a stable frequency, it is first necessary to ensure that the laser output by the semiconductor laser is a single longitudinal mode. In order to enable a semiconductor laser to output a single longitudinal mode, it is necessary to suppress other modes within the gain bandwidth and only allow one of the modes to be strengthened. The so-called external cavity is relative to the internal cavity. The internal cavity refers to the resonant cavity formed by the front and rear end faces of the semiconductor laser diode. The external cavity extends the resonant cavity to the outside of the semiconductor laser diode and uses optical feedback components to achieve external feedback. Thus forming optical oscillation, the grating is just a kind of optical feedback element to realize external feedback. Since the length of the external cavity is much longer than that of the internal cavity, the frequency interval between different laser modes oscillating in the external cavity is much smaller than that of the internal cavity. Once a single mode output is realized, the linewidth of the semiconductor laser will be greatly narrowed. There are usually two types of external cavity structures, Littrow and Littman, and their feedback elements are adjusted to change the length of the external cavity, so that a certain oscillation mode in the external cavity is selected to achieve a single mode output.

The Littrow structure external cavity semiconductor laser consists of three parts: a laser diode (the output surface is coated with an anti-reflection coating, and the other side is coated with a high-reflection coating), a collimator lens, and a grating, as shown in Figure 1. The output light of the laser diode is incident on the grating at a certain angle after passing through the collimating lens, and the appropriate incident angle is selected so that the output light of the grating is only 0-level and +1-level light, of which the 0-level light is used as the laser output light, and the +1-level light is It is fed back to the laser diode to form the external cavity oscillation of the semiconductor laser.

Littman structure external cavity semiconductor laser consists of four parts: laser diode, collimating lens, grating and reflector, as shown in Figure 2. In this structure, the output light of the laser diode enters the grating at a better angle through the collimator lens. Similarly, the 0-level light is used as the laser output light, and the +1-level light is used as the feedback light. The grating returns along the original path to form the external cavity oscillation of the semiconductor laser.

SUMMARY OF THE INVENTION

The purpose of the present invention is to disclose a tunable grating external cavity semiconductor laser based on prism beam expander. The laser has compact structure, simple assembly and adjustment, small intracavity loss and high gain, and can simultaneously obtain narrow line width and high output energy. index and achieve continuous tunability over a wide wavelength range.

In order to solve the above-mentioned technical problems and realize the above-mentioned technical effects, the technical solution one adopted in the present invention is:

A tunable grating external cavity semiconductor laser based on prism beam expansion, comprising a laser diode 11, a collimator lens 12, and a grating 14, is characterized in that it also includes a prism beam expansion system composed of two or more triangular prisms 13, so that The laser light emitted by the laser diode 11 passes through the collimating lens 12 and before passing through the grating 14, is dispersed and amplified by the prism beam expander system. The incident light angle of each triangular prism 13 is the Brewster's angle corresponding to the wavelength of the laser diode 11 . The outgoing light of each triangular prism 13 is evenly perpendicular to the outgoing edge of the triangular prism 13 .

The second technical solution adopted by the present invention is: a tunable grating external cavity semiconductor laser based on prism beam expansion, including a laser diode 21, a collimator lens 22, a grating 24, and a reflector 25, and is characterized in that it also includes several triangular prisms The prism beam expander system composed of 23 makes the laser light emitted by the laser diode 21 pass through the collimating lens 22 and before passing through the grating 24, be dispersed and amplified by the prism beam expander system. The incident light angle of each triangular prism 23 is the Brewster's angle corresponding to the wavelength of the laser diode 21 . The outgoing light of each triangular prism 23 is evenly perpendicular to the outgoing edge of the triangular prism 13 . The outgoing light of the prism beam expander system is the incident light of the grating 24, and the 1st-order diffracted light of the grating 24 is reflected back to the grating 24 by the mirror 25, and then returns to the laser diode 21 along the original path, and the 0-order light of the grating 24 is used as the external cavity of the tunable grating output of a semiconductor laser.

The beneficial effect of the present invention is: the tunable grating external cavity semiconductor laser based on the prism beam expansion, on the basis of the ordinary external cavity semiconductor, a prism beam expansion system is added in the optical path of the external cavity feedback, and the laser beam passes through the prism beam expansion system. , because the expansion of the beam increases the resolution of the grating, and the dispersion characteristics of the prism itself make the oscillating light in the outer cavity of the laser further dispersed, thereby further narrowing the linewidth of the laser.

Description of drawings

Fig. 1 is the schematic diagram of the principle of the existing Littrow type grating external cavity semiconductor laser line;

Fig. 2 is the schematic diagram of the principle of the existing Littman type grating external cavity semiconductor laser line;

Fig. 3 is a specific embodiment 1 provided by the present invention;

Fig. 4 is the second specific embodiment provided by the present invention;

Fig. 5 is a specific embodiment of the prism beam expander system provided by the present invention;

Fig. 6 is a schematic diagram of the expanded beam principle of the prism beam expander system of the present invention;

FIG. 7 is a comparison diagram of line width measurement in Embodiment 1 of the present invention (ie, FIG. 3 );

Fig. 8 is a comparison diagram of line width measurement of the second embodiment of the present invention.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Fig. 3 is a specific embodiment 1 provided by the present invention. As shown in FIG. 3 , it includes a laser diode 11 , a collimator lens 12 , a prism beam expander system, and a grating 14 . Among them, the laser diode 11 is used to output the laser beam, and under the action of the current, electrons in the conduction band and holes in the valence band recombine to generate stimulated radiation, and the inner cavity (the resonant cavity formed between the two end faces of the laser diode ) to resonate to generate laser light; the collimator lens 12 has a very short focal length and is used to shape the beam output by the laser diode 11 to form a parallel beam; the prism beam expander system is composed of several triangular prisms 13 and is used to change the Output parallel light beam spot size, improve system dispersion performance simultaneously, comprise two triangular prisms 13 in this specific embodiment; A resonant surface of the resonant cavity formed between the end face and the grating), the parallel beam output by the prism beam expander system hits the grating 14, and the first-order diffracted light returns to the laser after passing through the prism beam expander system and the collimator lens 12 in the original path The diode 11 oscillates and amplifies in the external resonant cavity of the cavity mirror at the output end of the grating 14 and the laser diode 11; and the 0-order light of the grating 14 is used as the output of the first embodiment.

Fig. 4 is the second specific embodiment provided by the present invention. As shown in FIG. 4 , it includes a laser diode 21 , a collimator lens 22 , a prism beam expander system, a grating 24 and a mirror 25 . Wherein, the function and positional relationship of the laser diode 21, the collimator lens 22, and the prism beam expander system are the same as those in Embodiment 1; Reflected by the reflector 25, it returns to the grating 24. The first-order light diffracted by the grating 24 passes through the prism beam expander system and the collimator lens 22, and then returns to the laser diode 21. The output end faces of the reflector 25 and the laser diode 21 are Oscillation amplification in the resonant cavity of the cavity mirror; the 0th-order light of the grating 24 is the output of the second embodiment.

Fig. 5 is a specific embodiment of the prism beam expander system provided by the present invention. As shown in Figure 5, the prism beam expander system is composed of four triangular prisms. The prism material of the prism beam expander system is fused silica or calcium fluoride, and the incident angle of each prism is the Brewster angle of the required wavelength laser. , the outgoing light is perpendicular to the other side of the prism.

Fig. 6 is a schematic diagram of the principle of beam expansion of the prism beam expander system of the present invention. As shown in Figure 6, the prism beam expander system includes a triangular prism 13, i ₁ and i ₂ are respectively the incident angle of the beam on the triangular prism 13 (that is, the Brewster angle corresponding to the wavelength of the laser diode 11) and the refraction angle, w _a , w and _b are respectively the cross-sectional dimensions before and after beam expansion, then the beam expansion coefficient M of the prism beam expander system is also the beam expansion coefficient of the triangular prism 13, which is:

Further extension to the beam expansion coefficient of the prism beam expander system made up of n triangular prisms 13 is:

Wherein, M ₁ , M ₂ ,···,M _n are beam expansion coefficients of prisms 1, 2,...,n respectively.

Referring to US20020186741A1, it can be seen that in a laser system combined with multiple triangular prisms and gratings, the full width at half maximum Δλ _FWHM of the line width of the output laser can be determined by the following formula:

Among them: θ _div is the divergence angle of the laser diode beam in the horizontal direction, M is the beam expansion multiple of the prism beam expander system, and α _B is the angle at which the laser is incident on the grating. _NR is the number of round trips of the laser in the resonator, and λ is the wavelength of the laser. It can be seen from the above formula that increasing the beam expansion coefficient M of the prism beam expander system can reduce the line width of the output laser and realize the narrowing of the line width of the laser.

FIG. 7 is a comparison diagram of line widths measured by a time-delayed self-heterodyne method according to a specific embodiment of the present invention. In this specific embodiment, the collimating lens 12 output diameter is the parallel light beam of 3.8mm; The prism beam expander system is made up of two triangular prisms 13, and the incident angle of the triangular prisms 13 is 56.5 °, and the beam expansion factor is 1.59, and the prism beam expander system The beam expansion factor is 2.53; the cavity length of the resonant cavity is 83.2mm; the length of the delay fiber used in the delay self-heterodyne measurement is 13.6Km. In FIG. 7 , the abscissa is the frequency interval, the ordinate is the signal strength, the dotted line represents the line width data of Embodiment 1 of the present invention, and the solid line represents the line width data of the original semiconductor laser. The 3dB attenuation bandwidth of the data reflects the true linewidth of the laser. The 3dB attenuation bandwidth of the first embodiment of the present invention is 19.4KHz, while the original semiconductor laser 3dB attenuation bandwidth is 44.3KHz. Therefore, the measured value of the line width in the specific embodiment of the present invention is 19.4KHz/2=9.7KHz, and at the same time, it can be obtained that the line width is narrowed by 44.3KHz/19.4KHz=2.28 times.

FIG. 8 is a comparison diagram of line widths measured by the time-delayed self-heterodyne method in Embodiment 2 of the present invention. In the second embodiment, the incident angle of the grating is 79°, and other parameters are basically the same as those in the first embodiment. In Fig. 7, the abscissa is the frequency interval, the ordinate is the signal strength, the dotted line is the second line width data of the specific embodiment of the present invention, and the solid line is the line width data of the original semiconductor laser. The 3dB attenuation bandwidth of the second embodiment of the present invention is 1.9KHz, and the 3dB attenuation bandwidth of the original semiconductor laser is 4.2KHz. Therefore, the measured value of the second line width of the specific embodiment of the present invention is 1.9KHz/2=9.7KHz, and at the same time, it can be obtained that the line width is narrowed by 4.2KHz/1.9KHz=2.21 times. It can be seen that the line width of Embodiment 2 of the present invention is narrowed by an order of magnitude compared with Embodiment 1, which is due to the narrowing of line width caused by the two diffractions of the grating.

While the present invention has been described in detail with reference to specific embodiments above, it is to be understood that the invention is not limited to the disclosed embodiments and that various changes in form and details will occur to persons skilled in the art. The above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of the present invention. within the scope of protection.

Claims (1)

1. a kind of saturating based on prism beam-expanded tunable grating external-cavity semiconductor laser, including laser diode (21), collimation Mirror (22), grating (24), reflecting mirror (25), which is characterized in that further include the prism beam-expanded system of several prisms (23) composition System；Laser diode (21) issue laser after collimation lens (22), by grating (24) before, by prism beam-expanded system Dispersion amplification；The incident light angle of each prism (23) is the corresponding Brewster's angle of laser diode (21) wavelength；Often The emergent light of a prism (23) is each perpendicular to the outgoing seamed edge of the prism (13)；The emergent light of prism beam-expanded system is grating (24) incident light, 1 grade of diffraction light of grating (24) is after reflecting mirror (25) is reflected back grating (24), along backtracking laser Diode (21), output of the 0 grade of light of grating (24) as tunable grating external-cavity semiconductor laser.