Laserdiode with integrated monitordiode

p0001The invention relates to a laser diode monolithically integrated PNP or NPN semiconductor layer structure pn with a laser active layer between the top two or np semiconductor layers on a substrate layer, a light outcoupling front facet, a light rear coupling rear facet and having a built-np or pn monitor diode with an area active monitor diode layer.

p0002In optical transmission modules, laser diodes are used almost exclusively in combination with a mostly in the back facet monitor diode arranged. This serves to measure the light emitted by the laser diode optical output power and thus to drive the laser driver circuit. The objective here is to ensure under all external changes, especially under conditions of temperature and age-related changes, constant optical output power of the laser at the front facet is. Simultaneously, the monitor diode can lasing suppress disturbing reflections from the rear facet. In a hybrid construction of monitoring and laser diode in addition to the manufacturing cost of such a monitor diode incurs additional costs in their assembly into the laser transmitter module.

State of the art

p0003From the <b>Publication"</b> Monolithically Integrated diplexer chip for PON Applications "by <nplcit id="ncit0001" npl-type="s"><text>A. Behfar et al. (Paper OTuM5, Optical Fiber Conference 2005, March 6-11, 2005, Anaheim, California, USA</text></nplcit>; also see the<patcit id="pcit0001" dnum="WO2005072224A2"><text>WO 2005/072224 A2 from 11.08.2005</text></patcit>) Is a pnp laser structure in the layer structure are known which combines a laser diode as a transmitter and a photodiode as a receiver for laser light of different wavelengths monolithically in a diplexer chip for passive optical networks (PON) in the formation of a Fabry-Perot laser and a monolithically having integrated monitor diode. The active laser layer is located between the top two pn layers. The light is coupled out vertically at a 45 ° from the laser active layer away sloping front facet. The monitor uses the diode laser active layer same as active monitor diode layer and is therefore connected to the laser diode optically in series. Monitor diode and the laser diode are separated by a vertical etch pit that extends into the n-layer from each other. The vertical etch pit reflects both as a light into the active laser layer rückkoppelnde rear facet. Part of the light generated in the active laser layer is in the monitor diode layer. The use of the laser active layer as a monitor diode layer but must be restricted to the Fabry-Perot type, since only here the emission spectrum and the absorption edge of the monitor diode layer have the same temperature dependence. In a DFB laser, the emission wavelength on the other hand shifts grid requires less with temperature than the absorption edge of the monitor diode layer, which with an uncooled operation leads to a disturbing, temperature-dependent sensitivity of the monitor diode in the field of monitoring at the wavelength.

p0004Furthermore, it is arranged at the diplexer chip known from the above publication beneath the laser active layer between the lower two np-layers an active-area photodiode layer, but which is part of the received photodiode. The receiving photodiode (1450 nm) (1310 nm) is designed for a different wavelength from the transmitting laser diode. A monitoring function within the meaning of the monitor diode can not be made to the photodiode, since an internal outcoupling of generated laser light would cause in the photodiode layer disturbing crosstalk between the different wavelengths of the transmitting and receiving signal, and thus is not permitted in the diplexer chip. Finally, in the known diplexers chip semüsiolierendes a substrate is used which is not conductive and therefore can not be used for a contact.

p0005All known approaches to integrating monitor diodes based on a horizontal series arrangement of laser diode and monitor diode with a sharing of the laser active layer as a monitor diode layer, but can be especially used in DFB lasers on a constant operating temperature.

task

p0006Based on the generic laser diode according to the above publication is the acknowledged <b>task</b> for the present invention seen in specifying such a laser diode with an integrated monitor diode, which can be implemented cost-effectively in various construction types. Here is the monitor diode, a temperature-independent measurement behavior show and do not interfere with the laser operation. The invention<b>solution</b> this problem is defined in the main claim. Advantageous developments of the invention are shown in the subclaims, which will be explained in more detail below in connection with the invention. In the laser diode according to the invention, the back facet has an inclination angle α greater than 0 ° and less than 90 ° with respect to the active laser layer at an angle α equal to 0 °. Thus, the incident in the rear facet of laser light is targeted and desired completely or partially downward toward the substrate layer reflects. Feedback in the laser active layer can be avoided or can be deliberately caused and adjusted by the selection of the inclination angle. Here, the chosen angle of inclination from the function of the application selected type of laser diode depends. A maximum light deflection results equal to or less than 45 ° at an inclination angle. For larger angles of inclination of reflected in the active laser layer light proportion is steadily increasing. The limit angles are 90 ° (vertical to the rear facet of laser layer) in which all the incident light is reflected into the laser and 0 ° (parallel to the rear facet of laser layer) in which no laser light can impinge on the rear facet. The monitor diode would be able to operate in these border cases only with scattered light.

p0007Below the laser active layer of the laser diode according to the invention is located parallel to the active monitor diode layer between the bottom two np or pn semiconductor layers, so that the light reflected by the rear facet down light enters the active monitor diode layer and is absorbed there. The monitor diode is thus connected optically parallel to the laser diode. The sequence of the two lower semiconductor layers thereby determine whether a NP or is designed to monitor a pn diode. The area monitor diode layer is formed at least in the area of ​​the rear facet, advantageous for a particularly simple production of the laser layer structure by depositing easy to extend however surface of the entire active laser layer. Compared to the laser diode, the pn between the top two or np semiconductor layers is connected in the forward direction for the power, the monitor diode is between the bottom two np or always connected pn semiconductor layers in the reverse direction for the current. It can thus both diodes are connected via a common branch without them influencing each other. Both diodes are electrically connected in series. The parallel monolithic integration of the monitor diode only marginal additional cost in the production of the laser diode are formed according to the invention. In addition there is a large module compactness, for example, compared with an optical series arrangement of laser and monitor diode is reached by the space savings is another advantage. The structural separation and optical parallelization of active laser layer and monitor diode layer in the laser diode according to the invention, the monitor diode operates independent of temperature in each laser design. Thus, a reliable measurement signal in the area of ​​optimal absorption is always generated.

p0008If the bottom semiconductor layer is advantageously formed at the same time as the substrate layer so that it is conductive, the integrated monitor diode can be contacted in a particularly simple manner. The monitor diode may then be contacted on the top and bottom of the laser diode electrically. In this case, the contact on the underside of the substrate layer is a simple surface contact. The opposite operation of the laser diode in the forward direction and the monitor diode in the reverse direction, a common n-type contact for both diodes can be used according to the invention in a PNP semiconductor layer structure is advantageous. In an npn-type semiconductor layer structure can be formed in accordance with a common p-type contact. Furthermore, both contacts of the laser diode on the upper side may be arranged, which means a manufacturing simplification and better interconnectability of the laser contacts. The contacts are then accordingly electrically isolated from each other. This configuration also allows a flip chip of the component.

p0009In an alternative design of the laser diode according to the invention additionally a vertical mirror with a depth of up to and including the laser active layer is integrated. This supports one hand reliable feedback in the laser active layer, so that increases the reflection component. Because the vertical mirror but extends only to the laser active layer, a light into the sloping rear facet on the p- underlying the laser active layer or n-type layer is still ensured, so that also in this embodiment, the monitor diode can be operated safely. It can be advantageously designed as a simple etching pit of vertical mirror. The applied flat upper-side contact of the laser diode can then also reach into these etch pit in order to increase the reflection effect of the vertical mirror. In addition, the rear facet can be mirrored by a continuation of the contact. To avoid short circuits, the contact with the etch pit and possibly. Also for the rear facet by a dielectric layer is isolated. As an alternative to this metallic mirror coating also an additional dielectric mirror coating to the rear facet and / or the vertical mirror may be used.

p0010A particular advantage of the laser diode according to the invention with integrated Monotordiode lies in the diverse constructive designability, so that different types of laser diodes can be realized. In this principle, the advantages of integrated monitor diode with respect to reliable signal generation and low manufacturing costs are preserved. First, can distinguish the laser diode type between a horizontal and a vertical light extraction. A horizontal light outcoupling is achieved when the front facet is arranged perpendicular to the resonator. however, the front facet has an inclination angle β greater relative 0 ° and less than 90 ° to the laser active layer at an angle β to equal to 0 °, a vertical light extraction upwards from the laser diode is achieved out. In the case of the inclined front facet both facets are at the laser diode according to the invention thus inclined and work as two acting in the reverse direction deflecting mirror. Preferred in this context is the angle of inclination of the rear facet and / or front facet between 90 ° and 60 °. Is on top of the laser diode, an opaque contact layer (laser contact) is applied, this has at the passage of light above the front facet on a corresponding window. Further, advantageously on the top surface of the laser diode, an antireflection coating may be applied can be avoided by the addition annoying reflections in the laser active layer.

p0011Furthermore, a laser diode according to the invention with a buried heterostructure (Buried Heterostructure, BH laser structure) or with a rib waveguide structure (Ridge Waveguide, RW laser structure) can be designed. Finally, in the laser active layer, a DFB or DBR grid (DFB Distributed Feed Back, DBR Distributed Bragg Reflector) be integrated, so that a DFB or DBR laser structure results. These structures show a further development of the Fabry-Perot laser structure has the advantage that monomodiges light having a constant wavelength is generated in dependence of the selected lattice parameters.

p0012Further embodiments of the laser diode according to the invention provide that the active laser layer, and the active diode layer each comprise a layer packet. This may be for example, an MQW structure (MQW multi quantum well) act, which is embedded in an optical waveguide. Finally, the layer structure to produce different emission wavelengths on an InP, GaAs, GaSb or GaN material system can be based. Depending on the material system, corresponding PNP or NPN layers are the consequences. In particular, the InP material system finds wide application in laser diodes.

p0013The DFB lasers previously used in optical signal transmission are involved in manufacturing is still very expensive. So laser bars are cleaved after processing a wafer with DFB lasers initially. To obtain a single-mode laser diode, at least one facet of the DFB laser must be non-reflective. For this, each individual laser bars must be handled with appropriate tools and placed in suitable holders ingots for installation in a Entspiegelungsanlage. After the anti-reflective coating, the laser bars are then removed again from the ingot holder. Bars, each individual laser is then characterized and made the selection based on the measurement results. Subsequently, the laser bars are broken into individual laser chips and laser diodes unsuitable sorted out. However, all described ingot processes are very time and labor-intensive and thus expensive.

p0014When with the laser diode resulting according to the invention with an integrated monitor diode new DFB laser diode with a vertical coupling of the laser light, the above-described ingot-handling can be completely avoided: The anti-reflective coating the laser facet occurs still on the wafer and the complete characterization and selection can using an automatic wafer prober done. The production cost of the proposed DFB laser diode with a vertical light extraction are therefore significantly lower than for standard DFB lasers. The "on-wafer" -Testbarkeit which hitherto deemed essential advantage of VCSEL structures, thus being equally transferable to "in-plane" laser structures. Compared to VCSELs but the optical output power are much higher in the laser diode according to the invention and problems in terms of spatial Multimodigkeit also consist not.

p0015In the literature, a variety of DFB lasers have been presented with vertical coupling so far. In most cases, however, it is DFB laser structures in which with the aid of a second-order DFB grating a coupling is achieved in the vertical direction. Alternatively, structures with vertical outcoupling DBR gratings were presented. The vertical coupling of the laser light by means of an integrated 45 degree mirror has also been proposed many years ago and is known for example from<b>publication</b> from <nplcit id="ncit0002" npl-type="s"><text>K. Iga et. al., "Surfac e emitting semiconductor lasers" (IEEE Journ. of Quantum electron., vol. 24, 1988, pp1845-1855</text></nplcit>) known. Such vertical light extraction through one of the laser active layer away sloping front facet in combination with a parallel inclined rear facet as deflection, an additional integrated monitor diode and an on-wafer antireflection the front facet, as it includes an embodiment of the laser diode according to the invention is, of this publication is not known, however. Therefore produces the laser diode according to the invention advantageous over known laser structures greatly simplifying the preparation, characterization and selection compared to conventional lasers by on-wafer technology by AR coating, characterization and selection can be made on the wafer; integration of a monitor diode without significant technological overhead and therefore cost reduction, compared to conventional laser diodes with vertical cavity significantly greater output power and a single-mode behavior even at high powers. The vertical monolithic integration is also space-saving, both in terms of per laser monitor diode device claimed wafer surface and in terms of space requirements in modules where compactness and so-called "foot-print" are becoming increasingly important.

embodiments

p0016Some embodiments of the laser diode with an integrated monitor diode according to the invention are described hereinafter for further understanding of the invention with reference to the schematic figures. In which:<dl id="dl0001" compact="compact"><dt><b>figure 1</b></dt><dd>a longitudinal section of a DFB laser diode having a vertical light extraction,</dd><dt><b>figure 2</b></dt><dd>a cross section through a laser diode with BH-DFB laser structure,</dd><dt><b>figure 3</b></dt><dd>Characterization diagrams of a laser diode with BH-DFB laser structure and vertical light extraction,</dd><dt><b>figure 4</b></dt><dd>a longitudinal section of a DFB laser diode having a vertical light extraction with additional metallic reflection and</dd><dt><b>figure 5</b></dt><dd>a longitudinal section of a DFB laser diode having a vertical light extraction with an additional vertical levels.</dd></dl>

p0017The <b>figure 1</b> shows a laser diode <b>LD</b> with a monolithically integrated semiconductor layer structure <b>HSA,</b> consisting of a first p-type semiconductor layer in the selected embodiment, it involves a p-type substrate layer <b>pS,</b> an n-type semiconductor layer <b>NHS,</b> another p-type semiconductor layer <b>pHS</b> and between the top two semiconductor layers <b>NHS, pHS</b> arranged laser active layer <b>LS</b> consists. In this embodiment, the active laser layer<b>LS</b> an additionally integrated DFB grating.

p0018Thus, there is a PNP semiconductor layer structure <b>HSA,</b> based on the InP material system in the selected embodiment. On the one hand, the semiconductor layer structure<b>HSA</b> from a light outcoupling front facet <b>FF,</b> on the other hand back-coupling light from a rear facet <b>RF</b> limited.

p0019The rear facet <b>RF</b> has an inclination angle α with respect to the laser active layer <b>LS</b> in a corner of α = 0 °. This angle of inclination α is between 90 ° (the limit of vertical rear facet<b>RF:</b> no light on a monitor diode layer <b>MDS;</b> maximum reflection for the laser diode <b>LD,</b> the monitor diode <b>MD</b> detects only the scattered light along the laser cavity) and 0 ° (the limit of horizontal rear facet <b>RF</b>). In the selected embodiment, an inclination angle α is represented by 45 °. The choice of the inclination angle α allows adjustment of the monitor diode layer<b>MDS</b> impinging laser light intensity and at the same time an adjustment of the laser diode for the <b>LD</b> effective reflection at the rear facet <b>RF.</b>

p0020Below the laser active layer <b>LS</b> is between the lower two np semiconductor layers, that is, between the n-type layer <b>NHS</b> and the p-type substrate layer <b>pS,</b> the active monitor diode layer <b>MDS</b> arranged. The monitor diode<b>MD</b> thus is optically parallel to the laser diode <b>LD</b> switched, whereas the electrical circuit between the two diodes <b>LD, MD</b> performed electrically in series. In the illustrated embodiment, the monitor diode extends<b>MD</b> not only in the rear facet <b>RF,</b> but area under the entire active laser layer <b>LS,</b> so it can be easily deposited during manufacture. That the PNP semiconductor layer structure<b>HSA</b> Laser generated light also passes to the inclined rear facet <b>RF</b> and is there a pro rata basis in accordance with the selected inclination angle α down to actively monitor diode layer <b>MDS</b> reflected (in the <b>figure 1</b> indicated by arrows). Through a corresponding contacting the monitor diode layer<b>MDS</b> is this a monitor diode <b>MD</b> formed, with the aid of the laser light power generated can be observed and controlled. In the selected embodiment, the monitor diode<b>MD</b> between a p-type contact Monitor <b>pMK,</b> of the large area of ​​the bottom of the p-type substrate layer <b>pS</b> is arranged and thus is also easy to manufacture, and the laser diode <b>LD</b> common n-type contact <b>GNK</b> (please refer <b>Figure 2)</b> on top of the laser diode <b>LD</b> contacted. The laser diode<b>LD</b> is between the common n-type contact <b>GNK</b> and a likewise arranged on the top p-type contact laser <b>pLK</b> contacted. Thus, the laser diode is<b>LD</b> in flow direction and the monitor diode <b>MD</b> operated in the reverse direction, so that both diodes <b>LD, MD</b> do not interfere with each other (cf. <b>figure 2</b>). Is an n-type substrate layer used (npn-type semiconductor layer structure<b>HSA),</b> then, according to a common p-type contact is used (in the <b>figure 1</b> not shown).

p0021In the <b>figure 1</b> is a laser diode <b>LD</b> presented with a vertical light extraction upwards. For this purpose, the front facet<b>FF</b> an inclination angle β greater than 0 ° and less than 90 ° with respect to the active laser layer <b>LS</b> at an angle β to equal to 0 °, so that the front facet <b>FF</b> inclined in the same direction as the rear facet <b>RF.</b> The inclination angle β is also between 90 ° (the limit of vertical front facet <b>FF)</b> and 0 ° (the limit of horizontal front facet <b>FF</b>). In the example described is also the angle of inclination β 45 °, so that both facets<b>FF, RF</b> parallel. However, since the rear facet<b>RF</b> the laser layer <b>LS</b> is inclined and the front facet <b>FF</b> away from, they reflect the laser light in a desired way down once and once upwards (in the <b>figure 1</b> indicated by arrows). On the upper side of the laser diode<b>LD</b> is a contact in the form of a p-type contact laser <b>pLK</b> applied, above the inclined front facet <b>FF</b> a window <b>FS</b> having.

p0022The limiting case of β = 90 ° to the front facet <b>FF</b> is in the <b>figure 1</b> indicated by a vertical dashed line. In this case, a horizontal light extraction reached (in the<b>figure 1</b> not shown).

p0023In the embodiment shown in <b>figure 1</b> is the front facet <b>FF</b> with an anti-reflection coating <b>AR</b> to avoid interfering coated back reflections. In addition, the rear facet<b>RF</b> an anti-reflection coating <b>AR</b> exhibit. To form a DFB or DBR laser diode, the active laser layer<b>LS</b> having an integrated DFB or DBR grid.

p0024In the <b>figure 2</b> (Not mentioned here are the reference numerals and the previous figures for details) is a cross section through the laser diode <b>LD</b> according to <b>figure 1</b> centrally between the facets <b>RF, FF</b> shown. It can be seen that the laser diode<b>LD</b> BH-structured with the buried active laser layer <b>LS</b> in a two etching pits <b>AB1, AB2</b> laterally delimited laser stripe <b>LN</b> is trained. On the laser stripe<b>LN</b> and the laser diode on the right of the right etching pit <b>STARTING AT 2</b> is the p-type contact laser <b>pLK</b> the laser diode <b>LD</b> to recognize. In the left etch pit<b>AB1</b> and the laser diode <b>LD</b> left of the left etch pit <b>AB1</b> is the common n-type contact <b>GNK</b> of laser diode <b>LD</b> and monitor diode <b>MD</b> arranged. Here, the p-type contact laser<b>pLK</b> and the common n-type contact <b>GNK</b> each by a dielectric insulating layer <b>DIS</b> from the non-contacting to the semiconductor layers <b>HS</b> electrically isolated. On the back of the p-type substrate<b>pS</b> is the p-type contact monitor <b>pMK</b> area arranged. On the laser stripe<b>LN</b> and through the two etching pits <b>AB1, AB2</b> away is the anti-reflection coating <b>AR</b> shown.

p0025Right next to the cross-section is in the <b>figure 2</b> the equivalent circuit diagram for the circuit of laser and monitor diode <b>LD, MD</b> both for a pnp-type semiconductor layer structure <b>HSA</b> as well as an npn-type semiconductor layer structure <b>HSA</b> shown. It can be seen that the laser diode<b>LD</b> always in the direction of flow and the monitor diode <b>MD</b> is always operated in the blocking direction for the current and the two diodes <b>LD, MD</b> not interfere with each other. Furthermore, it can be seen that once an np and once a pn diode monitor<b>MD</b> results.

p0026In the <b>figure 3</b> is the characterization of a laser diode produced <b>LD</b> in accordance with DFB grating according to the invention BH-structured and vertical light extraction <b>Figures 1 and 2</b> shown. In the top diagram, the optical output power of an L = 300 microns long laser diode<b>LD</b> (Pulse output <b>PAL</b> in mW) on the current (current <b>I</b> in mA) at an operating temperature <b>T</b> of 20 ° C indicated (solid curve). The variation of voltage<b>V</b> in mV across the current shown (dashed curve). The corresponding optical spectrum (wavelength<b>WL</b> in nm on relative intensity <b>RI</b> in dB) at a current <b>I</b> = 50 mA is shown in the lower diagram. The side mode suppression of the laser diode<b>LD</b> is more than 45 dB.

p0027In the <b>figure 4</b> (Not mentioned here numerals are respectively the previous figures refer to) is a laser diode <b>LD</b> shown, in which the metallization of the p-type contact laser <b>pLK</b> about the rear facet <b>RF</b> extends. This additional metallization leads to an increase of the reflectivity of the rear facet<b>RF</b> in terms of mirroring. On the front facet<b>FF</b> in turn, is an anti-reflection coating <b>AR</b> provided.

p0028The <b>figure 5</b> (Not mentioned here numerals are respectively the previous figures refer to) shows a laser diode <b>LD</b> the integration of an additional vertical mirror <b>VSP</b> before the rear facet <b>RF</b> for achieving controllable reflection. A part of the laser light still reaches the rear facet<b>RF</b> and is downward on the monitor diode <b>MD</b> distracted. In the illustrated embodiment, the additional vertical mirror<b>VSP</b> formed as metallized etch pit provided with a dielectric insulating layer <b>DIS</b> against the top two pn or np semiconductor layers <b>HS</b> is electrically insulated. On both facets<b>RF, FF</b> in turn, is an anti-reflection coating <b>AR</b> provided.

LIST OF REFERENCE NUMBERS

p0029<dl id="dl0002" compact="compact"><dt><b>FROM</b></dt><dd>etch pit</dd><dt><b>AR</b></dt><dd>Antireflection coating</dd><dt><b>DIS</b></dt><dd>dielectric insulating layer</dd><dt><b>FF</b></dt><dd>front facet</dd><dt><b>FS</b></dt><dd>window</dd><dt><b>GNK</b></dt><dd>common n-conductive contact</dd><dt><b>LD</b></dt><dd>laser diode</dd><dt><b>HSA</b></dt><dd>Semiconductor layer structure</dd><dt>I</dt><dd>electricity</dd><dt><b>LN</b></dt><dd>laser stripes</dd><dt><b>LS</b></dt><dd>active laser layer</dd><dt><b>MD</b></dt><dd>monitor diode</dd><dt><b>MDS</b></dt><dd>Monitor diode layer</dd><dt><b>NHS</b></dt><dd>n-type semiconductor layer</dd><dt><b>PAL</b></dt><dd>Pulse output</dd><dt><b>pHS</b></dt><dd>p-type semiconductor layer</dd><dt><b>pLK</b></dt><dd>P-type laser Contact</dd><dt><b>pMK</b></dt><dd>p-type contact Monitor</dd><dt><b>pS</b></dt><dd>p-type substrate layer</dd><dt><b>RI</b></dt><dd>relative intensity</dd><dt><b>RF</b></dt><dd>rear facet</dd><dt><b>T</b></dt><dd>Operating</dd><dt><b>VSP</b></dt><dd>additional vertical mirror</dd><dt><b>WL</b></dt><dd>wavelength</dd><dt>α</dt><dd>Tilt angle RF</dd><dt>β</dt><dd>Tilt angle FF</dd></dl>