2.1

Module body conceptual approach

The conceptual approach to the new generation of laser modules rests on versatility and multi-functionality, embodied by a design incorporating two arbitrary semiconductor-based active or passive chips and two optical ports that can be used as input or output ports according to the requirements, see Figure 1. For instance, a MOPA would incorporate a laser chip or even an ECDL, an amplifier chip, and two optical output ports, one behind the rear facet of the laser chip and the other in front of the output facet of the amplifier. The same approach can now be used to build, for example, a doublestage amplifier, consisting of a pre-amplifier and a main amplifier. The optical port behind the entry facet of the preamplifier then serves as input port, channeling light from an external laser into the system. After passing through the preamplifier and main amplifier, the laser light exits the system through the optical port in front of the main amplifier. Table 1 lists a series of possible combinations of active and passive semiconductor chips that can be implemented in the present laser module concept.

Figure 1.

CAD Model of an ECDL-MOPA module with dimensions 30 x 80 x 10 mm³. VHBG: volume holographic Bragg grating; μ-isolator: micro-optical isolator; μ-mirror: micro-mirror. The optical fibers exiting the fiber coupler are not shown in the figure.

Table 1.

Selection of possible combinations of active and passive semiconductor chips for implementation into a module.

Figure 1 shows a CAD model of the module body, in this case with chip 1 constituting the gain chip of an ECDL and chip 2 an amplifier. The module body has dimensions of 80×30×10 mm³ and features a mass of about 50 g. A thick AlN ceramic plate serves as a base plate and ensures mechanical stability of the assembly. Above, a thinner, intermediate AlN ceramic plate and finally three narrow AlN ceramic rails together set up the platform used to implement the electrooptical system. The AlN ceramic plates are assembled by flux-free soldering. Patterns of metallization on the AlN ceramic plates are realized with lithographic methods well established for electronic hybrids so that each of the AlN ceramic plates features an electrical functionality similar to what is provided by printed circuits boards. Thanks to the excellent heat conductivity of AlN ceramics even small electric strips feature large ampacity. Integration of through-vias into the AlN plates allows for stacking and hence for implementation of rf-compatible, high ampacity, multi-layer electric layouts. Stacking also provides the means to implement more complex 3D mechanical geometries.

The laser diode chips, each of which are either soldered or adhesively bonded on an AlN submount, are integrated inside the space between the rails by adhesive bonding. The intermediate AlN plate and the three rails, together with mechanical structures cut-out inside the AlN, also serve as anchor points for the attachment of the optical components. Electrical connections between chips and the AlN ceramic module body are implemented by conventional Au-wire bonding.

The general principle behind the optical path on the module body is explained via the example of the ECDL-MOPA shown in Figure 1. However, the basic principles behind the beam shaping and beam collection at the optical ports remain the same for all other implementations mentioned in Table 1. For the example discussed here, the local oscillator is implemented by an ECDL, whose resonator is formed by the front facet (facing towards chip 2) of chip 1 and by the effective mirror plane of the volume holographic Bragg grating (VHBG). Intra-cavity lenses collimate the beam exiting the rear facet of chip 1 onto the VHBG, thus creating a stable resonator for the ECDL. Two laser beams exit the ECDL, a main local oscillator beam at the front facet of chip 1 and an auxiliary beam at the rear facet of the VHBG.

The main local oscillator beam is collimated and travels through a μ-isolator before being focused into the waveguide of chip 2, a single-pass power amplifier. The μ-isolator ensures that no stray light, no directly reflected light, and no spontaneous emission of chip 2 is fed back into the local oscillator. At the output of chip 2, the amplified light is collimated before being deflected twice with the help of μ-mirrors positioned at 45° to the beam axis. The beam is subsequently injected into a fiber coupler ferrule, consisting of a coupling lens and a single-mode, polarization-maintaining fiber whose facet lies in the focal plane of the lens. A polarizing beam splitter in front of the fiber coupler filters out the fraction of the optical field oscillating in the undesired polarization. It should be noted that, although a cutout for a μ-isolator is foreseen between the two μ-mirrors, none is actually implemented. This is because the only μ-isolators existing in the present form factor (∅4 mm, 5 mm length) have a maximum rating that is well below the intensities provided by the output of chip 2. However, until now, no perturbation of the local oscillator due to feedback into the amplifier has been observed. Depending on the application one has in mind, a fiber-coupled isolator may be added outside the module to provide isolation of the module from optical feedback created by the environment.

The beam exiting the ECDL through the VHBG is, per resonator design, quite collimated. This auxiliary beam path is very similar to that of the main beam emitted by chip 2, except that here a μ-isolator is placed between the two μ-mirrors in order to avoid feedback into the local oscillator through the auxiliary port.

All optical elements are adhesively bonded to the ceramics using UV-cured epoxy adhesives. Since micro-lenses with small focal lengths (from a few 100 μm to a few mm) are used, the position of the components must be maintained during curing of the adhesive with some 100 nm. This ensures that beam aberrations and clipping are minimized and that optimal collimation or focusing is achieved. When it comes to coupling into the fiber coupler (fiber facet in the focal plane of the coupling lens), the main constraint is the angle between the beam and the optical axis of the coupler. To first approximation, the angular misalignment between the laser output of chip 2 and the fiber coupler is solely determined by lateral misalignment of the lenses collimating the output of chip 2. In practice, it has been observed that lateral misalignments of lenses in the sub-micrometer range are sufficient to cause a reduction of 10% in the coupling efficiency into the optical fiber.

The bonding technology applied to the micro-lenses on the laser module body does not enable gap-free bonding. As a consequence, misalignment of the optics cannot be avoided during curing, when the adhesive shrinks. At present, the combination of adhesives that are in use and of the geometry and tolerances of the AlN body and of the micro-optical elements the misalignment through shrinkage can be limited to approximately 1 μm. This is definitely not sufficient if optimal fiber coupling efficiency is targeted. Therefore, the micro-mirrors are used to compensate the residual misalignment of the micro-lenses. Although the micro-mirrors are also adhesively bonded, a special, gap-free fixation method developed at the FBH considerably reduces the residual bonding-induced misalignment. We estimate the residual effective misalignment to be well low 100 nm.

The electrical layout of AlN ceramic body allows for operation of chips that feature sections. This provides significant functional versatility: at the position of chip 1, for example, a DBR laser with separate control of the injection current to the Bragg reflector section and to the gain section can be used. Alternatively, a DBR laser chip with integrated heater for the Bragg reflector section could be operated. Also, DBR or Fabry-Perot chips with a long gain section and a short (and fast) modulation or reverse bias voltage section could be integrated (the latter if mode-locking is targeted). At position of chip 2, for example, tapered amplifier chips with separate control of the injection current to the ridge waveguide input mode filter/preamplifier and to the tapered section may be used.

A photodiode is implemented behind the mirror just in front of the main port fiber coupler. This provides to monitor the power injected onto the fiber coupler.

To provide ultra-fast access to the laser diode injection current, wide bandwidth (10 GHz) transistors can either be integrated on the rails or – for the highest frequency requirement – directly on the laser chip submount. In various setup we have demonstrated a 3 dB-bandwidth between 1 and 3 GHz with respect to power modulation. This eases frequency control of the master oscillator and power control at the power amplifier.

To ease thermal control and monitoring of the laser module, a minimum of 3 thermistors in integrated. One is buried with the ceramic body to determine the temperature of the ceramic module body. Each of the chip submounts carries its own thermistor very close to the laser chip so that the chip temperature can be closely monitored.

To provide flexibility for the thermal management a micro-thermo-electric-cooler (TECs) can replace the submount for the laser chip at chip position 1 (TEC.Chip). This allows for temperature control of chip 1 with a time constant of about 0.1 s and thermally decouples chip 1 from the ceramic module body. This is important for implementation of MOPAs based on monolithic master oscillators: here, the chip 1 temperature has to be set to control the emission frequency, while the power amplifier at chip position 2 requires the ceramic module body temperature to be as low as possible to provide the best power efficiency and the highest output power possible. This μTEC also carries a thermistor.

For ECDL applications another micro-TEC can be integrated into the module body to allow for thermal tuning of the VHBG (TEC.VHBG). This TEC carries a thermistor for closed-loop temperature control. The thermistor is buried inside the TEC to reduce thermal cross-talk from the environment.

2.2

Packaging technology

The conceptual approach to the implementation of the housing is based on technology well established for electronic hybrids.

The housing material is Kovar, a ferromagnetic Fe-Ni-Co alloy that features a modest heat conductivity of approx. 17 W/(K·m) a Young’s modulus of approx. 150 GPa and a coefficient of linear expansion of approx. 5 ppm/K at room temperature and well above. For the latter it is very well established for integration with AlN (coefficient of linear expansion approx. 4.7 ppm/K at room temperature and well above) in the field of electronic hybrids. Further, also as heritage from electronic hybrid technology, a glass-soldering technology exists that allows for integration of electrical feedthroughs into the housing. The housing shown in Figure 2 features a form factor of 175×75×23 mm³ and a mass of approximately 750 g.

Figure 2.

ECDL-MOPA laser module for operation at 780 nm. The photograph shows the AlN ceramic body, the housing as well as the optical and electrical feedthroughs.

We have developed isolated coaxial feedthroughs that provide an ampacity of 3 A direct current (DC) and an rf signal bandwidth well above 10 GHz in a 50 Ω environment. Galvanic isolation from the housing helps to avoid electrostatic damage to the device, specifically to the laser diodes, and it eases measures against electro-magnetic interference (EMI). Each signal transduced from the inside to the outside of the module or vice versa is interfaced through its own isolated coaxial feedthrough. For high current applications, feedthroughs can be paralleled by proper Au-wire bonding on the ceramic module body.

In order to transmit the optical signals between inside and outside the module we selected an approach that relies on implementing fiber coupling already on the ceramic module body. An alternative, more common approach would have been to implement the fiber coupling unit (fiber and focusing lens) within the housing. However, this requires high mechanical stability between the ceramic module body and the housing. For example, relative beam pointing between the laser beam generated on the ceramic module and the optical axis of a fiber coupler integrated into the housing has be kept well below 100 μrad which we believe cannot be guaranteed with existing module designs in a realistic application environment. The requirements become even more stringent when smaller wavelengths are addressed.

We hence use a Kovar ferrule with a diameter of 4 mm that carries a pre-aligned focusing lens and a single mode, polarization maintain optical fiber. This rigid assembly is integrated into the ceramic module body. The optical fiber is fed through a flange attached to and sealed against the housing. The fiber itself is sealed against the flange by means of a low outgassing adhesive. The feedthrough maintains a polarization extinction ratio of more than 20 dB as long as the fiber is not under mechanical stress inside the module.

When all hybrid integration work has been completed the module housing is closed by seam welding with lid made of Kovar.