CN105006742A - Wavelength thermal tuning device of external cavity semiconductor laser and synchronous thermal tuning method - Google Patents

Abstract

A wavelength thermal tuning device and synchronous thermal tuning method of an external cavity semiconductor laser, the wavelength thermal tuning device includes a semiconductor gain chip, a Bragg grating and a temperature controller, along the laser output direction of the semiconductor gain chip is the described The Bragg grating, the semiconductor gain chip and the Bragg grating are placed on the temperature controller. The invention makes the thermal drift of the longitudinal mode of the laser cavity consistent with the thermal drift of the reflection spectrum of the grating through the design of the temperature sensitivity of the external cavity light path. When the temperature changes, the longitudinal mode distribution and the reflection spectrum of the grating drift synchronously, and the laser maintains the original working performance, thereby realizing wide-range wavelength tuning. The invention improves the wavelength tuning ability of the existing narrow line width external cavity semiconductor laser.

Description

Wavelength Thermal Tuning Device and Synchronous Thermal Tuning Method for External Cavity Semiconductor Laser

technical field

The invention relates to an external cavity semiconductor laser, in particular to a wavelength thermal tuning device and a synchronous thermal tuning method of the external cavity semiconductor laser. Mainly used in optical communication, optical sensing and other fields.

Background technique

Due to its unique chip structure, semiconductor lasers have the advantages of direct photoelectric conversion, small size, long life, and high integration. They are widely used in optical communications, optical storage, and optical sensing. However, the linewidth of semiconductor lasers is usually relatively wide, for example, the linewidth of common DFB lasers is on the order of megahertz. It greatly limits its application in coherent optical communication, fiber optic sensing and other fields.

One of the most mature methods to achieve narrow linewidth output is to use an external cavity laser, that is, to extend the resonant cavity of the laser to passive devices outside the chip (such as gratings, FP etalons, etc.). Ordinary external cavity lasers can achieve a linewidth of tens to hundreds of kilohertz. A wide range of wavelength tuning can be achieved by changing the incident angle of the grating through the mechanical structure. However, the number of components in the external cavity is large (usually including volume gratings, lenses, mirrors, and mechanical movement devices, etc.), the optical path of the external cavity is long (greater than 1cm), and the size of the laser is greatly increased. The Tunable External Cavity Lasers sold by Thorlabs are 250x200mm in size.

At present, the smallest and most compact external-cavity narrow-linewidth semiconductor laser in the industry is the PLANEX series [ACHIEVING LOW PHASE NOISE IN EXTERNAL CAVITY LASER IMPLEMENTEDUSING PLANAR LIGHTWAVE CIRCUIT TECHNOLOGY, US 20100303121A1] [SUDEVICEFABRICATION WITH PLANAR EF ARGRAT S PLANAR BR AGG] [ACHIEVING LOW PHASE NOISE IN EXTERNAL CAVITY LASER] from RIO. , US8358889]. They adopted the method of direct butt-joint connection between the silicon dioxide planar waveguide Bragg grating and the laser chip, using the short external cavity and the negative feedback principle of the hypotenuse of the grating, and realized the narrow line width output of 2-10kHz in the 14pin butterfly package. . The mechanical size is only 26x15mm. However, its wavelength tuning is realized by heating the entire external cavity laser, and the tuning range is small, about 20G.

Another 14PIN butterfly package miniaturized external cavity laser comes from Emcore (formerly K2Photonics, later acquired by Emcore). He uses fiber Bragg grating hypotenuse negative feedback to achieve 30KHz linewidth, low 1/f noise, RIN<-155db/Hz [Achieving narrow linewidth, low phase noise external cavity semiconductor lasers through the reduction of 1/f noise, K2Optronics, Proc.of SPIE Vol.6133,613301(2006)]. But it is also a fixed-frequency laser without wavelength tuning function.

The first two technologies are miniaturized 14PIN butterfly package external cavity semiconductor technologies, but their wavelength tuning capabilities are very weak, both on the order of tens of GHz. Achieving both narrow linewidth and wide range tuning in a miniaturized package is challenging. Emcore's narrow-linewidth tunable lasers make this possible. This is currently the most widely used or even the only product in coherent optical communication (the product line was acquired by Neophotonics in 2014), which is based on the original Intel external cavity tunable laser technology [Thermally tuned external cavity laser with micromahined silicon etalons : design, process and reliability. In Proceedings of ECTC, 2004.818-823]. The mode selection filter of the device is two cascaded Fabry-Perot (F-P) etalons made of silicon material, using the vernier effect, so that only two etalons transmit the peak-to-peak wavelength The coincident longitudinal modes can be oscillated, while the other longitudinal modes are suppressed. By precisely controlling the peak wavelength of the transmission peak of the etalon through temperature, wavelength tunability can be realized. The tuning range of this product can cover C-band or L-band, and any wavelength can be tuned within the tunable range. The linewidth is 20kHz, and the Side mode suppression ratio (Side mode suppression ratio, SMSR) is 55dB. Emcore has successfully Integrate with the modulator and launch the corresponding products. There are no moving parts in this design, and the stability is good, but in order to achieve precise wavelength tuning, precise temperature control of the two silicon etalons is required, which is somewhat difficult. And the application in optical communication is generally to switch the wavelength from one channel to another. If a large range of continuous tuning of the wavelength is required, complex circuit control is required.

Contents of the invention

The object of the present invention is to provide a wavelength thermal tuning device and a synchronous thermal tuning method of an external cavity semiconductor laser. The device and method can make the miniaturized external cavity semiconductor laser meet the tuning performance of narrow line width and large wavelength at the same time. And the tuning mechanism is simple and can be tuned continuously.

The core idea of the present invention is: by designing the temperature sensitivity of the external cavity light path, the thermal drift of the cavity longitudinal mode of the laser is consistent with the thermal drift of the grating reflection spectrum. This enables large-scale simultaneous thermal tuning.

Technical solution of the present invention is as follows:

A wavelength thermal tuning device for an external cavity semiconductor laser, characterized in that it includes a semiconductor gain chip, a Bragg grating and a temperature controller, the laser output direction along the semiconductor gain chip is the Bragg grating, and the semiconductor gain chip The gain chip and the Bragg grating are placed on the temperature controller.

The Bragg grating is a fiber Bragg grating, the fiber optic head of the fiber Bragg grating has a ground conical lens opposite to the semiconductor gain chip, and the end of the gain chip near the fiber Bragg grating is coated with an anti-reflection coating. The two reflective surfaces of the laser resonator are composed of the outer end face of the semiconductor gain chip, that is, the end face far away from the grating and the Bragg grating. The Bragg grating is placed on the heat-sensitizing device, and the semiconductor gain chip and the The heat-sensitizing device is placed on the temperature controller, and the semiconductor gain chip, fiber Bragg grating, temperature controller and heat-sensitizing device are packaged in a box.

The Bragg grating is a silicon waveguide grating, and the laser output from the output end of the silicon waveguide grating is coupled into the output fiber through a focusing lens. The semiconductor gain chip, silicon waveguide grating, temperature controller, focusing The lens and one end of the output optical fiber are packaged in a box body 5 .

The above synchronous thermal tuning method for external cavity semiconductor lasers is characterized in that the thermal drift of the longitudinal mode of the laser cavity matches or approaches the thermal drift rate of the grating reflection spectrum through the heating of the temperature controller.

The coupling between the semiconductor gain chip and the fiber grating is achieved by grinding a tapered lens on the fiber tip. The tapered lens can couple the output light of the gain chip into the optical fiber within a short distance (less than 50um).

Since the mode fields of the silicon waveguide Bragg grating and the semiconductor gain chip waveguide are similar (or the mode fields of the two are matched through the tap gradient silicon waveguide design), the coupling between the silicon waveguide Bragg grating and the semiconductor gain chip can be realized by direct chip docking. Coupling of the SiWBG to the external fiber can be achieved through a discrete lens or a tapered lens ground on the fiber.

Features and advantages of the present invention are:

The invention has a simple structure and can realize miniaturized packaging, such as 14PIN butterfly packaging. Narrow linewidths are also achieved using the hypotenuse reflectance spectral feedback method. Through the special design of the temperature sensitivity of the external cavity optical path, large-scale synchronous thermal tuning is realized. The tuning mechanism is simple and does not require complex circuit control. Continuous tuning is possible.

Description of drawings

Fig. 1 is the external cavity semiconductor laser device diagram of embodiment 1 of the present invention adopting thermally-sensitized fiber Bragg grating

Figure 2 is an illustration of the principle of wavelength synchronous thermal tuning

Fig. 3 is an external cavity semiconductor laser device diagram using a silicon waveguide Bragg grating in Embodiment 2 of the present invention

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings, but the protection scope of the present invention should not be limited thereby.

First please refer to Fig. 1, Fig. 1 is the external cavity semiconductor laser device diagram that the present invention adopts thermally-sensitized fiber Bragg grating, as can be seen from the figure, the wavelength thermal tuning device of the external cavity semiconductor laser of the present invention comprises semiconductor gain chip 1, and the output light passes through the optical fiber The conical lens 6 with its end face ground is coupled into the fiber Bragg grating 2 . The end surface of the gain chip 1 close to the grating is coated with an anti-reflection film to eliminate the resonant cavity effect of the gain chip itself. The other end is coated with a high reflection film, which is used to form a resonant cavity with the external cavity grating. Since the tapered lens 6 is directly ground and processed, it is integrated with the optical fiber and has a compact structure, which can realize the coupling of light beams in a very short distance (less than 30 microns). The fiber Bragg grating is thermally sensitized by means 4, so that the center wavelength shift of its reflection spectrum is more sensitive to temperature. The fiber grating thermal sensitization device 4 needs to achieve a high thermal tuning rate in a compact space. Here the fiber grating is encapsulated in a medium with a large expansion coefficient (organic polymer). The thermal expansion of the medium will stretch the optical fiber, thus changing the central wavelength of the reflection peak. The existing experimental results have obtained a temperature drift coefficient of 72pm/K, which is 6.5 times of the temperature drift coefficient of the fiber grating itself. The temperature controller 3 performs temperature-variable operation on the laser to realize wavelength thermal tuning. The above components are encapsulated in the box body 5 .

The principle of wavelength synchronous thermal tuning is shown in Figure 2. The solid line curves and arrows represent the reflection spectrum of the grating and the longitudinal mode distribution of the laser resonator before temperature adjustment. The circle represents the laser operating point, that is, the position of the lasing wavelength. The laser works on the hypotenuse of the grating reflection spectrum, not at the reflection peak, which is conducive to narrowing the line width [ACHIEVING LOW PHASENOISE IN EXTERNAL CAVITY LASER IMPLEMENTED USING PLANAR LIGHTWAVE CIRCUITTECHNOLOGY, US 20100303121A1]. However, narrow linewidth can only be achieved in a small area of the hypotenuse. If the longitudinal mode of the cavity drifts away from this area, there will be performance degradation such as reduced optical power and widened linewidth.

The performance of thermal wavelength tuning depends on the following parameters: the temperature drift coefficient Da of the semiconductor gain chip, the temperature drift coefficient Gg of the Bragg grating, the ratio of the optical path length of the semiconductor gain chip and the optical path length of the Bragg grating to the total optical path length of the resonator are respectively Ra, Rg. Among them, because the air gap is very small and can be ignored, Ra+Rg=1. The overall thermal drift rate of the output wavelength of the laser when the temperature changes (that is, the thermal drift of the external cavity longitudinal mode) D_total can be written as:

D_total=RaDa+RgDg

It can be seen from the above formula that for fixed Ra, Rg, Da, increasing Dg, that is, increasing the thermal sensitivity of the grating will help to increase the thermal tuning rate D_total of the laser. On the other hand, the relative drift rate D_diff between the external cavity longitudinal mode and the grating reflection spectrum brought about by the temperature change can be calculated as follows:

D_diff=D_total-Dg=Ra(Da-Dg)

It can be seen from the above formula that for fixed Ra, Rg, Da, reducing the difference between Da and Dg is the only way to reduce the relative drift. Because the temperature drift coefficient Da of the semiconductor gain chip is relatively large, about 96pm/K, and the temperature drift coefficient Dg of the silicon dioxide material of the fiber grating is relatively small, about 11pm/K, there is a big difference between the two. Therefore, once the two are heated at the same time, the drift of the longitudinal mode of the external cavity and the drift of the reflection spectrum of the grating will be out of sync. In turn, the laser deviates from the working area and performance deteriorates.

Assuming that the wavelength range of the working region of the laser on the hypotenuse of the grating is B, the allowable temperature range T=B/D_diff, and the total wavelength tuning range of the laser W=T*D_total=B*D_total/D_diff.

Embodiment 1 of the present invention is to increase the temperature drift coefficient Dg by thermally sensitizing the grating to make it reach a level equivalent to that of the semiconductor gain chip Da. On the one hand, the relative drift rate D_diff is reduced, and on the other hand, the overall drift rate D_total is increased. When the temperature changes, the longitudinal mode distribution and the grating reflection spectrum drift synchronously, as shown by the dotted curve and the arrow in the figure. This ensures that the laser works in the original working area and realizes large-scale thermal tuning. Taking Ra as 0.2 and Dg as 72pm/K into the calculation, it can be obtained that the overall wavelength drift rate D_total increases to 2.74 times of the original, and the relative drift rate D_diff decreases to 0.28 times of the original. By dividing the two, the total wavelength tuning range W will be 9.8 times that of the unsensitized fiber grating external cavity.

Embodiment 2 of the present invention is realized by using a silicon waveguide Bragg grating, as shown in FIG. 3 . As can be seen from the figure, the wavelength thermal tuning device of the external cavity semiconductor laser of the present invention includes a semiconductor gain chip 1, a silicon waveguide grating 7, and a temperature controller 3, and the laser output direction along the semiconductor gain chip 1 is the silicon waveguide Grating 7, focusing lens 8 and output optical fiber 9, described semiconductor gain chip 1 and described silicon waveguide grating 7 are placed on described temperature controller 3, described semiconductor gain chip 1, silicon waveguide One end of grating 7 , focusing lens 8 and output optical fiber 9 is packaged in a box body 5 .

Because the silicon waveguide and the gain chip are both III-IV materials, the temperature drift coefficient is similar, and Dg and Da are similar, both of which are between 95pm/K and 105/K. The relative drift rate D_diff can be reduced to less than 10pm/K, and the overall wavelength thermal drift rate D_total can be increased at the same time. The total wavelength tuning range W will be nearly 30 times that of ordinary fiber grating external cavity lasers. The fabrication of silicon waveguide Bragg gratings can be achieved by etching microgrooves [Fabrication and Characterization of Narrow-Band Bragg-Reflection Filters in Silicon-on-Insulator Ridge Waveguides, JLT 2001]. Since the mode fields of the silicon waveguide Bragg grating 7 and the semiconductor gain chip 1 are similar in size (or the mode fields of the two are matched through the tap gradient silicon waveguide design), the coupling between the silicon waveguide Bragg grating and the semiconductor gain chip can be realized through direct chip docking . The coupling between the silicon waveguide grating and the external optical fiber 9 can be realized through a separate lens 8, or a tapered lens optical fiber can also be used.

Claims (4)

1. a wavelength thermal tuning device of external cavity semiconductor laser, it is characterized in that: comprise semiconductor gain chip (1), Bragg grating (2) and temperature controller (3, along the laser of described semiconductor gain chip (1) The output direction is the Bragg grating (2), and the semiconductor gain chip (1) and the Bragg grating (2) are placed on the temperature controller (3). 2. the thermal wavelength tuning device of external cavity semiconductor laser according to claim 1, is characterized in that: described Bragg grating (2) is fiber Bragg grating (2), on the fiber head of this fiber Bragg grating (2) Grinding tapered lens is arranged opposite to described semiconductor gain chip (1), and described gain chip (1) is coated with anti-reflection film near the end of fiber Bragg grating (2), and the two reflective surfaces of this external cavity laser resonant cavity are made of The outer end face of the semiconductor gain chip, that is, the end face away from the grating and the Bragg grating, the Bragg grating (2) is placed on the heat-sensitizing device (4), the semiconductor gain chip (1) and the The heat-sensitizing device (4) is placed on the temperature controller (3), and the semiconductor gain chip (1), fiber Bragg grating (2), temperature controller (3) and heat-sensitizing device (4) be packaged in a box body (5). 3. the wavelength thermal tuning device of external cavity semiconductor laser according to claim 1, is characterized in that: described Bragg grating (2) is silicon waveguide grating (7), the output of described silicon waveguide grating (7) The laser output from the terminal is coupled into the output optical fiber (9) through a focusing lens (8), and the described semiconductor gain chip (1), silicon waveguide grating (7), temperature controller (3), focusing lens (8 ) and one end of the output optical fiber (9) are packaged in a box body 5. 4. the synchronous thermal tuning method of claim 2 or 3 described external cavity semiconductor lasers, it is characterized in that: by the heating of described temperature controller (3), make the thermal drift of laser cavity longitudinal mode and grating reflection spectrum The thermal drift rate matches or approaches.