DE4432794A1 - Semiconductor laser module for coupling to optical fibre - Google Patents

Abstract

The laser module includes a semiconductor laser (1) in a housing, double lens coupling optics system (5,20), and an isolator (6). The double lens couples light into an optical waveguide, esp. a mono-mode optical fibre (31), while the isolator lies in the beam path between the two coupling lens, a monitor diode (8), and a Peltier-effect cooling device. Provided in the housing is a micromechanically structured silicon carrier plate (2) formed through etching. The carrier plate provided in the housing has one slot (3) to accommodate one coupling lens (5) and the isolator. Another slot (4) is aligned with the first slot and is used for holding the monitor diode. A cylindrical imaging microlens couples the laser to a mono-mode fibre (31) via the coupling lens optics.

Description

The invention relates to a laser module with a Semiconductor laser in a housing, a two-lens Coupling optics for coupling to an optical fiber, a Isolator in the beam path between the two lenses and with a Peltier cooler.

Such laser modules are from EP 02 59 888 B1 and DE 42 32 327 A1 known. A laser is opened in both writings described a chip carrier, the light with a first Lens is collimated and the housing wall over an oblique angle posed plan window penetrates. The housing will be there hermetically sealed with the plan window. Outside the housing, the light is turned on with a second one Focused lens on the face of an optical fiber. In the second document there is also an isolator described the outside of the hermetically enclosed Volume sits in a nozzle that fits into the housing protrudes and carries the window at its end.

A disadvantage of the known solutions is that the Laserchip sits on a chip carrier, which itself with geometric tolerances. These tolerances together with the tolerances of the laser chip itself and the Mounting tolerances of the laser chip on the chip carrier and the Determine chip carrier on a mounting plate in your Interact the position of the light emitting surface of the  Lasers. Because the beam waist of the emerging from the laser Beam that approximates as a Gaussian beam elliptical profile can be described Has waist half axes of less than one micrometer, the Position of the first lens to the laser in the submicrometer range are exactly fixed. To achieve this accuracy, in the solutions according to the state of the art an adjustment of the Fiber is provided in the axial and lateral directions. Of the Tolerance range for the position of the laser to the first lens is thereby to a volume of approx. 20 · 20 · 20 µm³ expanded. This extended tolerance range is lateral Direction through the maximum permissible tilt angle of approx. 1 ° the axis of the light beam to the lens axes and the Fiber axis and the allowable beam offset of the collimated beam to the focusing lens. To Large tilt angles as a result of an excessive lateral offset lead between laser and collimation lens in particular with aspherical lenses to a coma-like distortion of the Beam. An excessive axial misalignment between the laser and the collimating lens leads to a change of the enlargement ratio, which leads to coupling losses compared to the optimal magnification ratio. To the another becomes the beam in the area of the focusing lens widened more, so that the lens edge or already the isolator located between the lenses is part of the Beam cuts off, which also lead to coupling losses would.

The extended tolerance range of the laser can also be used the assembly process according to the state of the art without adjustment not or only with very high costs and effort. In the previously known solution DE 42 32 327 A1 are therefore several adjustment options specified. On the one hand triaxial active adjustment of the collimation lens to the first  Laser. Then a lateral adjustment of the second lens serves at the same time to adjust the beam direction the focused beam below the diagonally cut fiber the right angle. In addition, there is a three-axis Adjustment of the fiber provided.

In the previously known document EP 0 259 888 B1 the Laser diode also mounted on a chip carrier. This is used for pre-positioning in grooves on a carrier plate in a plane perpendicular to the beam direction relative to one Lens shifted, which is also in this carrier plate is mounted. As the laser moves during a active adjustment must be in operation, you get problems with the connection lines of the laser and with the heat dissipation. Even after the fixation, the chip carrier only lies with its Side surface to a side surface of the groove in the Carrier plate on, resulting in significant thermal resistance between chip carrier and carrier plate and thus to one insufficient laser cooling.

In both previously known solutions, in addition to fiber adjustment still a complex and costly adjustment between the laser and the first lens. In the previously known solution DE 42 32 327 A1 is between the lenses another isolator placed in the collimated beam to avoid disturbing effects on the laser. Such Isolators work according to the Faraday principle and have in a narrowly specified wavelength and temperature range a very high return loss which, depending on the type (single stage or two-stage) can be 30 to 60 dB. You leave however the specified temperature or Wavelength range, the return loss decreases sharply, which can cause the laser to malfunction. The constancy the wavelength of the laser is in the previously known solution  through a Peltier control with thermistor temperature measurement reached near the laser. But the isolator is in one Mounted socket, which is embedded in the housing wall. When the housing temperature changes, it also changes the temperature of the insulator with. If the Ambient temperature or the laser power is the Temperature of the laser constant via the Peltier control held. Thereby the laser gets warmth over the Peltier element withdrawn and fed to the housing. Since the isolator with the Housing in thermal contact make itself there Temperature fluctuations are even more noticeable, thereby reducing the isolator's damping properties deteriorate.

Another problem is that for coupling a Semiconductor laser to a single-mode fiber an imaging Coupling optics is required, as is usually the case Field distribution of a laser not with that of a fiber matches. A single mode fiber usually has one almost Gaussian mode field with a radius of approx.4.5 µm and circular symmetry. A semiconductor laser has typically an elliptical mode field with a large one Semi-axis of approx. 2 µm and a small semi-axis of cs. 0.5 µm. Must be a laser with a strongly elliptical far field the mode fields of the lasers can be used and the fiber with a rotationally symmetrical optic is not fully map to each other, making the maximum achievable coupling efficiency is limited.

It is an object of the invention to provide a laser module which the disadvantages described above are avoided.

The task is carried out by a laser module with the characteristics of Claim 1 solved. Advantageous further developments are  specified in the subclaims.

The present solution according to the invention avoids the above Defects of the known solutions described and leads other improvements. In the present solution is only an active adjustment step on an uncritical one Point, namely required on the optical fiber side, which is also required for the previously known solutions is. Here the required tolerances are in the range of 2 µm in the lateral directions and of 30 µm in axial direction. These tolerances can be known flange adjustment method and with fixation with Adhere to laser welding spots.

To adjust the elliptical mode field of the Semiconductor laser relies on the circular Moenfeld of the fiber an imaging lens with a cylindrical component in order for the two semiaxes of the laser mode field to achieve different magnifications. So that cylindrical microlens need not be adjusted, the first groove that for the first lens and the isolator is provided, a widening, for example, a has a flat groove bottom. This broadening is can be produced by anisotropic etching. The cylindrical Microlens can be inserted into the broadening and is due to the sloping side walls in this centered.

An exemplary embodiment of the laser module according to the invention will be explained with reference to the drawings.

Show it:

Fig. 1 is a plan view of an inventive laser module with butterfly package,

Fig. 2 is a longitudinal section through the above laser module,

Fig. 3 shows an enlarged detail of Fig. 1,

Fig. 4, the holder for the laser alignment marks,

Fig. 5 to 7 plan view, side view and view of the first lens with the insulator in the inserted state.

Figs. 1 and 2 show in plan view and longitudinal section according to the invention a laser module. The laser chip 1 is mounted directly on a micromechanically structured silicon carrier plate 2 . This carrier plate 2 is shown on a larger scale in FIG. 3. The silicon carrier plate 2 has two anisotropically etched grooves 3 and 4 . The groove 3 serves to receive the collimation lens 5 and the isolator 6 . In addition, the groove 3 has a widening. This broadening is produced by anisotropic etching. It has a flat groove base and is arranged on the laser-side end of the groove 3 . In this widening there is a cylindrical microlens 40 , which serves to adapt the elliptical mode field of the semiconductor laser 1 to the circular mode field of the fiber 31 . The cylindrical microlens 40 is held in the widening of the groove 3 by the widening being dimensioned such that the microlens 40 rests on the bottom of the groove and is adjusted by the inclined side walls. An aspherical lens can preferably be used as the collimating lens in order to avoid spherical aberrations and thus to improve the coupling efficiency. The lens 5 has a cylindrical edge 7 or a mount with a cylindrical edge 7 . The outside diameter of the lens 5 or the lens holder is preferably as large as the outside diameter of the likewise cylindrical insulator 6 . However, it is also possible for the lens and isolator to have different outside diameters, in which case the groove 6 must have different widths in the areas for the lens mount and for the isolator mount. The groove 4 serves to receive the monitor diode 8 , which is required for regulating the power of the laser chip 1 . This monitor diode 8 is mounted on the oblique rear wall 9 of the groove 4 facing away from the laser. The laser 1 sits on the web 10 remaining between the grooves 3 and 4 . The width of the web corresponds approximately to the length of the laser. In the case of lasers whose radiating zone lies on the top of the chip (epi-up), it is advantageous to choose the width a of the web 10 to be just as large or a few micrometers larger than the chip length 1 of the laser chip, since then even with any assembly tolerances Base area of the laser is in good thermal contact with the silicon substrate everywhere. In the case of lasers whose radiating zone is on the underside (epi-down), on the other hand, it is advantageous if the web width a is the same size or a few micrometers smaller than the chip length, so that the beam is not shadowed by the silicon substrate in the event of a mounting tolerance.

According to the prior art, laser chips are mounted on heat sinks with end faces perpendicular to the mounting surface. In the case of epi-down lasers, the edge between the front end face and the mounting surface must be as sharp as possible, so that on the one hand there is good heat transfer between the laser chip and heat sink and on the other hand the laser beam is not shadowed by the heat sink. However, the vertical limitation of the heat sink according to the prior art constricts the heat flow from the laser via the heat sink, as a result of which the heat resistance is increased even in the case of highly conductive heat sinks such as, for example, diamond. In the solution according to the invention, however, the side surfaces of the grooves 3 and 4 located in front of the laser mirrors are inclined by the angle of repose α = 54.7 ° relative to the substrate surface due to the geometry of the anisotropic etching of silicon. On the one hand, this angle is large enough so that no light can be shadowed even with lasers with a very wide far field. On the other hand, the angle is much smaller than 90 ° as in the heat sinks according to the prior art. As a result, the heat flow lines can spread beyond the base area of the laser chip, which leads to a reduction in the thermal resistance. The edges produced by a highly precise anisotropic etching process between the surface of the substrate serving as the mounting surface and the side surfaces of the grooves 3 and 4 are very sharp and their position is highly precise. This makes it possible to adapt the width a of the web 10 to the length 1 of the laser chip with a tolerance of a few micrometers and thus to achieve good thermal contact between the laser chip and the silicon substrate without the risk of beam shadowing.

FIG. 4 shows an enlarged section from the central area of the web 10 , which is provided for receiving the laser 1 . On both sides of the receiving surface is a V-groove 11 and 11 'anisotropically etched, which serve as marks for aligning the laser chip. During assembly, the laser is positioned between these two marks under visual observation. The laser does not have to be in active operation as in the prior art, but a passive adjustment is sufficient here, which can be carried out much more easily since the laser does not yet have to be contacted. With the passive adjustment according to the marks 11 and 11 ', a positioning accuracy of a few micrometers is possible. Since the marks 11 and 11 'are produced in the same anisotropic etching process as the grooves 3 and 4 , their mutual position tolerance is in the range of less than 2 micrometers. This gives a very precise alignment between the laser and the collimating lens 5 and the isolator 6 .

The position of the collimation lens 5 is clearly defined by the position and size of the groove 3 . The thickness d of the silicon substrate 2 is selected depending on the radius of the collimating lens so that the contacting surface line still lies on the side flanks of the groove 3 . In Figs. 5, 6 and 7, the inserted lens insulator in three section planes with top view, shown from the side and from the front. In order to achieve a small working distance between the laser and the collimating lens, the frame of the collimating lens is conically shaped in its front part and thus adapted to the oblique end flank of the groove 3 . The height of the center of the lens must be adapted to the height h of the radiating surface of the laser. The height of the lens center can be adjusted via the groove width b of the groove 3 . The slope angle α is calculated based on the crystallographic properties of the silicon

α = arctan (√2) = 54.7 ° (1)

The groove width b, which is required so that the center of the Lens with an outer radius r is at height h

b = r · √6 - h · √2 (2)

The slope lines touching the cylinder surface lie in a depth of

m _y = r / √3 - h (3)

and have a distance of

m _x = r · √8 / 3 (4)

from each other.

For a lens radius of, 1500 µm and a height of optical axis over the substrate surface of h = 100 µm for epi-up lasers you get

b = 3533 µm
m _y = 766 µm
m _x = 2449 µm

With h = 0 for epi-down lasers one gets

b = 3674 µm
m _y = 866 µm
m _x = 2449 µm

Since the contacting surface lines lie at a depth of 766 µm or 866 µm below the substrate surface, the substrate must have at least this thickness so that the contact lines are still within the side surfaces of the groove and not on the lower edge. A substrate thickness of 1000 µm is sufficient to hold the lens. The groove must then be completely etched through, with the lens protruding slightly below the underside of the substrate. The mounting plate 41 located on the Peltier element 42 under the silicon carrier plate must have a corresponding recess in the region of the groove. This recess can be omitted if the silicon carrier plate is thicker than the lens radius. A high-precision axial stop for the collimation lens is obtained through the end face of the groove 3 . In this way, the collimation lens can be positioned without adjustment in all three coordinates with the high accuracy that can be achieved by silicon micromechanics relative to the laser. The fixation can be done by a known method, such as gluing. It is advantageous here that the adhesive gap has a width of zero in the support line and therefore no misalignment can occur when the adhesive shrinks.

The silicon carrier plate 2 also serves to accommodate the electrical contact lines for the bearing 1 and the photodiode 8 . For high-frequency applications, an insulation layer, for example made of polyimide, can be applied in a thickness of 10-20 μm on the silicon surface. This insulation layer serves as a dielectric for HF microstrip waveguides or for coplanar HF waveguides. In microstrip waveguides, there is a full-area metallization layer as a mass between the silicon substrate and the insulation layer. With coplanar RF lines, the electrical waveguide 13 is surrounded by two lateral ground lines 14 and 14 '. In addition, other components can be mounted on the silicon substrate. In this way, the thermistor 15 can be mounted directly near the laser on the silicon substrate with good thermal contact with the laser. Capacitors, resistors and coils, which are required for the HF control of the laser or even an integrated control module, can also be mounted on the silicon substrate in the immediate vicinity of the laser. Since in the solution according to the invention the HF lines can be led directly to the laser diode, frequencies of up to 30 GHz can be achieved. In contrast, according to the prior art, if the HF lines of control circuits have to be routed outside the housing via housing bushings and bonding wires to the laser, only about 2-3 GHz can be achieved.

In the solutions according to the prior art, a slanted plan window is brought into the beam path between the lenses for hermetically sealed light transmission. In the exemplary embodiment, the focusing lens 20 is hermetically sealed or welded into the opening 21 of the housing end wall 22 . For welding, the focusing lens is provided with a weldable metal frame. For soldering, the lens also has a metal frame with a solderable coating or the solderable coating is applied directly to the edge of the unmounted lens. As a result, the housing 23 can be considerably less expensive than the housings that are required for designs according to the prior art, since instead of the socket with the oblique plan window, only one opening 21 has to be made in the housing end wall 22 . The lens 20 focuses the light beam on the end face 30 of a fiber 31 . To avoid back reflections, the end face 30 can be inclined with respect to the optical axis. The fiber 31 must then be prepared according to the refraction of light at the inclined end surface is also at an oblique angle in a guide sleeve 31st With this guide sleeve, which runs in a socket 33 , the fiber can be adjusted axially. For lateral adjustment, the end face of the sleeve 33 is displaced on the end face of a spacer sleeve 34 . The axial and lateral positions are preferably fixed by laser welding. The axial and lateral adjustment and the fixation can advantageously be carried out in one operation with automatic control. This automatable work step is the only active adjustment step that is required in the solution according to the invention. The tolerances are so large that misalignment during laser welding has only a very slight influence on the coupling efficiency.

Claims (9)

1. Laser module with a semiconductor laser in a housing, with a two-lens coupling optics for coupling light into an optical waveguide, in particular a single-mode fiber, with an isolator in the beam path between the lenses of the coupling optics, with a monitor diode and with a Peltier cooler, characterized in that
that a silicon carrier plate ( 2 ) is provided in the housing, which is micromechanically structured by means of anisotropic etching technology,
that a first groove ( 3 ) is provided which serves to receive the first lens ( 5 ) and the insulator ( 6 ),
that a second groove ( 4 ) is provided in alignment with the first groove ( 3 ) and serves to receive the monitor diode ( 8 ),
that the semiconductor laser ( 1 ) is mounted on the web ( 1 ) between the ends of the grooves ( 3 , 4 ) on the silicon carrier plate ( 2 ),
that a cylindrical microlens ( 40 ) is provided between the semiconductor laser ( 1 ) and the first lens and that a widening of the first groove ( 3 ) is provided, into which the cylindrical microlens ( 40 ) is inserted. 2. Laser module according to claim 1, characterized in that on the web ( 10 ) V-grooves ( 11 , 11 ') are provided, which serve as marks for aligning the semiconductor laser ( 1 ). 3. Laser module according to one of claims 1 or 2, characterized in that the monitor diode ( 8 ) is attached to the inclined side wall of the groove ( 4 ) remote from the laser ( 1 ). 4. Laser module according to one of claims 1 to 3, characterized in that the width of the web ( 10 ) is the same size or slightly larger than the length of the laser ( 1 ) if it has its radiating zone near the top or that the width of the Web ( 10 ) is the same size or slightly smaller than the length of the laser ( 1 ) if it has its radiating zone near the bottom. 5. Laser module according to one of claims 1 to 4, characterized in that on the silicon carrier plate ( 2 ) electrical contact lines, components RF microstrip waveguide and / or control circuits, components are provided. 6. Laser module according to one of the preceding claims, characterized in that the optical waveguide ( 31 ) is fastened to the outside of the housing in an adjustable manner. 7. Laser module according to claim 6, characterized in that in the housing between the insulator ( 8 ) and optical waveguide ( 31 ) an inclined window or the second lens ( 20 ) is hermetically sealed in the housing wall. 8. Laser module according to one of the preceding claims, characterized in that the silicon carrier plate ( 2 ) is attached to a Peltier cooler ( 42 ). 9. Laser module according to one of the preceding claims, characterized in that the cylindrical microlens ( 40 ) is designed such that the elliptical mode field of the semiconductor laser is adapted to the circular mode field of the optical waveguide through microlens ( 40 ) and the lenses of the coupling optics.