WO2002073756A2

WO2002073756A2 - Optical devices

Info

Publication number: WO2002073756A2
Application number: PCT/GB2002/001145
Authority: WO
Inventors: Richard Vincent Penty; Ian Hugh White; Laurence John Sargent; Aeneas Benedict Massara
Original assignee: University Of Bristol
Priority date: 2001-03-12
Filing date: 2002-03-12
Publication date: 2002-09-19
Also published as: WO2002073756A3; GB0106075D0; GB2373369A

Abstract

A laser diode device comprises a layered structure, and defines two upper contact regions to which bias current signals can be applied. A first contact region receives a fist bias signal and a modulating signal. Such a device can produce a light output signal which can be modulated at high frequency using a low modulating current swing and can operated in uncooled conditions, whilst giving satisfactory output characteristics compared with single contact devices.

Description

OPTICAL DEVICES

The present invention relates to optical devices, and to laser diode devices in particular.

BACKGROUND TO THE INVENTION

Local area network data rates are set to increase to 2.5Gb/s and lOGb/s. Data rates can also be expected to increase beyond this level. The recent growth in demand for data communications has fuelled interest in techniques for reducing the cost of laser sources. For a low-cost, high-speed data link to be realised, reliable uncooled laser operation at a wavelength of, for example, 1.3μm would provide an attractive solution. A major challenge here is the design of an uncooled source, which possesses sufficient bandwidth at elevated temperature for the desired data rate. Another key issue in considering the source as a whole is to keep the drive currents required to operate the device at high speed as small as possible to reduce heating effects and to be compatible with the low voltage/current swings achievable with high speed electronics. It will be desirable for transceiver designs to make use of low supply voltages, such as 3.3V or 2V. The use of such low voltages places stringent demands on the modulation capabilities of sources, and as such, current designs of low-cost drivers could struggle to provide more than 40mA modulating current swing at 85°C.

It is therefore desirable to provide a laser device which can operate in an uncooled environment, and which can provide high speed, low current operation. Such a device can therefore operate at low drive voltages . SUMMARY OF THE PRESENT INVENTION

According to the present invention, there is provided a method of modulating a light output of a laser diode which has an active region for producing a light output, and first and second contact regions for supplying respective bias signals to the active region, the method comprising: supplying a first bias current signal to the first contact region; supplying a second bias current signal to the second contact region; and supplying an RF modulating current signal to the first contact region, wherein each of the first and second bias current signals serves to forward bias at least part of the active region of the laser diode.

One particular embodiment of the present invention provides a laser device with a ridge waveguide, which has been etched fully across the central ridge to produce a two-contact device. Such a device can provide an increased small signal bandwidth over previously-considered single contact device performance.

Such a two-contact device can facilitate a large signal direct-modulation regime, which allows enhanced high temperature operation. With previously-considered large signal direct- modulation techniques, large modulation and bias currents are required in order to achieve the required modulation speed and bandwidth. With a device embodying one aspect of the present invention, low bias and modulation currents can be used to provide suitable modulation speed and bandwidth. An example of such a device shows clear open eye diagrams at lOGb/s modulation up to a temperature of 85°C with a low current swing of only 10mA using a laser structure that would otherwise only operate at 70°C, potentially easing transceiver electronics specifications. An extinction ratio of 6dB is measured for current swings of only 30mA. This compares with 80mA which is required for typical previously-considered single contact laser devices. It is emp'hasised that the term "comprises"' or "comprising" is used in this specification to specify the presence of stated features, integers, steps or components, but does not preclude the addition of one or more further features, integers, steps or components, or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: - Figure 1 shows a plan photograph of an example laser device embodying one aspect of the invention;

Figures 2 and 3 are - plan and cross-sectional schematic views respectively of the device of Figure 1;

Figure 4 shows a schematic side view of the device of Figure 1;

Figure 5 illustrates power to bias characteristics at room temperature for the device of Figures 1 to 4;

Figure 6 illustrates comparison of small signal -3dB bandwidths for single and two-contact lasers at room temperature and 85°C;

Figure 7 illustrates back-to-back eye diagrams at lOGb/s at

(a) 25°C and (b) 85°C and (c) 25°C for the device of Figures 1 to

4;

Figure 8 is a plan photograph of another laser device in accordance with the invention;

Figure 9 is a schematic plan view of the device of Figure

8; Figure 10 illustrates performance of the device of Figures 8 and 9;

Figure 11 is an eye diagram for the device of Figures 8 and 9 ; and

Figure 12 is an output spectrum graph for the device of Figures 8 and 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a plan photograph of an example laser device which embodies one aspect of the present invention. Figure 2 is a schematic plan view of the device of Figure 1, and Figure 3 is a schematic cross-sectional view through section A-A in Figure 2. In this particular example, the device 1 is an InGaAsP-InP laser, with a ridge-waveguide 2. The device active region comprises seven ,0.8% compressively strained quantum wells, and operates at a centre wavelength of 1.3μm. The laser is, in this instance, 280μm long, and acts as a Fabry-Perot (F-P) laser. It is cleaved on an output facet 3, and can also be cleaved or, for example, HR coated to 88% on a rear facet 4. The laser is bonded junction side up on a temperature-controlled submount (not shown) and the output can be connected via a lensed fibre (not shown) .

The laser diode device shown in Figure 3 comprises a substrate material in which an active region 5 is contained. The upper surface of the substrate is etched to form a ridge waveguide structure. The ridge waveguide structure 2 comprises a pair of parallel etched channels arranged either side of the central ridge. The device 1 has a P⁺ contact layer on the top surface of the substrate. ^' The etched structure carries a silicon dioxide insulating layer 7, which itself carries a metal layer 8. As can be seen from Figure 2, a trench 9 is etched across the device, i.e. across the width of the ridged waveguide channels, so as to divide the metal layer 8 into two distinct .regions. The first of these regions provides a first contact region 10 and the second provides a second contact region 11.

The central ridge waveguide can have a width of, for example approximately 3 μm, with etched channels of width x of approximately 8 μm either side of the ridge waveguide.

Such a structure is generally known and understood,, and so a more detailed description of it will be omitted here for the sake of clarity. It will be readily appreciated that the structure described is merely one example of laser devices to which the present invention can be applied.

Figure 4 illustrates a side schematic cross-sectional view of the device of Figure 1, which shows the active region 5.

In a method embodying one aspect of the present invention, a first DC bias current signal I_a is applied to the first contact region 10, to bias part of the active region 5 of the device 1. A second DC bias signal lb is supplied to the second contact region 11. Each of the first and second DC bias signals serves to forward bias a respective part of the active region 2 of the device 1 below the contact region concerned. In addition to the bias signal I_a, a radio frequency (RF) modulating signal is supplied to the device through the first contact region 10. In one particular example device the two contact-regions are produced by using focused ion beam etching (FIBE) of a standard single contact laser device so that the single contact is divided in two by a trench. Such a technique allows direct comparison between double and single contact devices to be made. The etching removes the metal, silicon dioxide and contact layers (8, 7, 6 respectively). In this manner the contacts can be split to give a first contact region providing a desired modulation section length. The ratio of first contact region length to second contact region length can be any desired value, for example 2:7. The first contact region can be 40μm for example. In such an example, resistance between the first and second contact regions is measured to be in excess of 200Ω, for a 5μm wide etch.

In this illustrated embodiment of the invention, the contact separation etch takes place fully across the width of the device as shown in Figures 1 and 2. The modulation section provided by the first contact region can be located towards either of the front 3 and rear 4 of the device.

The post-etch performance of the device can be characterised in terms of the CW L-I_b characteristics at room temperature (25°C) . Figure 6 illustrates these characteristics for an example device for a range of values of first bias current I_a and as a function of second bias current I_b. As can be seen, a hysteresis is observed for I_a=0mA. In this illustrated embodiment of the invention, with the first region (the short section) biased with a first bias current of I_a=10mA, the threshold current for the device to begin lasing is reduced from 40mA (at I_a=0mA) to around 16mA indicating that the etched trench does not have any particularly adverse affect on the device. The pre- and post-etch slope efficiencies remain unchanged. The small signal performance of the laser device (at an r.f. power of -lOdBm) is illustrated in Figure 7 for increasing values of d.c. bias current at room temperature. In Figure 7, this is characterised in terms of the -3dB bandwidths as a function of d.c. bias current (the second bias current I_b for the twin contact device) . In this example, at room temperature, the slope gives a modulation current efficiency of 1.37GHz/ "VmA for the single contact device. The two-contact laser, with the short modulation section (first contact region 10) biased at 10mA, gives an increased modulation efficiency of 1.64GHz/ "VmA. A -3dB bandwidth in excess of 15GHz is found for the two-contact device as compared to a maximum of 10GHz for a single contact device. At elevated temperatures a -3dB bandwidth in excess of 7GHz is still found at 85°C for the twin-contact device, with a modulation current efficiency of 0.93GHz/ ^•mA giving a significant improvement over uniform modulation at this temperature where the bandwidth is seen to clamp at 5GHz. The first contact region (short section) of the device has increased differential gain compared to the single contact design. Higher differential gain then leads to increased bandwidth capability.

In order to investigate the suitability of the device as a source for data communications application, direct NRZ (nonreturn to zero) modulation at a data rate of lOGb/s (PRBS = 2²³- 1) is carried out. Figures 7a, 7b and 7c show respective eye diagrams for illustrating the performance ^' of the device. With the long section (second contact region) of the device biased with a second bias current of I_b=65mA, open eyes are clearly obtained as can be seen from Figure 7a. What is significant however is that the short modulation section is biased with a first bias current of I_a=5mA and has a peak-to-peak current swing of only 10mA applied. The resultant eyes are very clean with little noise being evident. This high quality pattern indicates that the modulation technique can be used for high speed devices, whilst employing desirably low drive currents. ^' Figure 7b shows the back-to-back eye diagram at 85°C, again for a peak-to-peak modulation current swing of 10mA, with I_a biased at 10mA. The eye can clearly be seen to be open, although with some increased noise over the previous conditions. In contrast, previously-considered devices show fully closed eye diagrams at temperatures above 70°C.

The results for the example device described thus far have been for the monitored light output from the long section (second contact region) (as illustrated in Figures 1 and 2). Alternatively, the light output could be produced from the short section (first contact region) . Figure 7c illustrates and eye diagram for the short section light output measured at lOGb/s modulation as before. An extinction ratio of 6dB is measured for a 30mA peak-to-peak current swing being applied (I_a=15mA) . The eye diagram of Figure 7c is still very clear which indicates that transceiver drive electronics specifications could be eased using such a device. For comparison, a peak-to-peak current of 80mA is required to obtain an extinction ratio of >6dB for the single contact device. This demonstrates that more than a 60% reduction in required current swing can be achieved in devices embodying the present invention.

The enhanced small signal performance of the two-contact scheme hence facilitates modulation at increased temperatures, in contrast to the previous single contact devices which operate up to only 70°C under uniform NRZ modulation.

The techniques described above can also be applied to single mode laser devices, for example devices that make use of distributed reflector structures such as the distributed feedback laser (DFB) or distributed Bragg reflector (DBR) laser.

Figures 8 and 9 illustrate another laser device embodying an aspect of the present invention. The device is similar in structure to that described above with reference to Figures 1 and 2, but includes a wavelength selective reflector to each side of the central ridge waveguide 2.

One particular example reflector can be obtained by etching into the active layer on either side of the waveguide. This has the advantage that the reflector can be obtained without using additional regrowth steps. The reflector can take any suitable form. By way of non limiting examples, a distributed Bragg reflector (DBR) structure or distributed feedback (DFB) structure can be used. The reflector could alternatively be provided by a grating pattern, which could be any ID, 2D or 3D configuration, in any pattern which provides a suitable reflector.

In one example of this second device, the distributed reflectors can take the form of an etched 2D-lattice grating. The grating is etched into respective bottom surfaces of the channels 9. In one particular example, the grating has a length L of approximately 50μm, and is located towards the front facet 3 of the device, for example approximately ~50μm from the front facet 3. The length and positioning of the grating is not material for the purposes of the present invention. Figures 9 and 10 illustrate the possible positions of the grating patterns or arrays 14 which can be etched into the channels 9 on either side of the ridge waveguide 2. Each array 14 comprises a series of holes, etched through the top layers to a depth which is comparable to the depth of the active region 5 of the device. In this illustrated embodiment, the holes are arranged in a hexagonal array. That is, each hole away from the edge of the array is surrounded by six equally spaced holes. This separation, can be, in one particular example, of the order of 0.6-0.65μm, and the holes have a diameter d such that the diameter-to-pitch ratio d:a is 0.33, in this embodiment of the invention.

Figure ^' 10 illustrates the small signal performance, characterised in terms of the -3dB bandwidths as a function of the second dc bias-current, l - At room temperature, the slope gives a modulation current efficiency of 1.36GHz/VmA for the single contact device. The two-contact device, with the short section (first contact region) biased at 10mA gives an increased modulation efficiency of 1.60GHz/VmA. A -3dB bandwidth of around 15GHz is found for the two-contact device as compared to a maximum of 10GHz for the single contact device. This can be further enhanced by increasing the split ratio of the contacts .

Figure 11 illustrates an eye diagram for the two-contact device.

In order to investigate the suitability of the two-contact device embodying the invention as a source, direct NRZ modulation at a data rate of lOGb/s (PRBS=2²³-1) is carried out. An extinction ratio of 6dB is found for only a 30mA peak-to-peak current swing being applied (I_a=15mA) with the eye diagram clearly open (Figure 11) . For comparison, a peak-to-peak current of 40mA is required to obtain an extinction ratio of >6dB under uniform modulation. This demonstrates that a 25% reduction in required current swing is achieved.

The optical spectrum shown in Figure 13 illustrates clearly that such a device lases in a single longitudinal mode. Purely single-mode operation is maintained over the entire operating current range up to over 3 times threshold. Typical SMSR values of >24dB are obtained and the dynamics are clearly not perturbed by provision of the grating. AR coating the back facet 4 may facilitate improved SMSR values.

One possible use of the invention is in an integrated circuit package (or chip) which has a number of, for example four, such laser sources. The distributed reflector structures of the four sources can be different to one another, such that the overall package can selectively provide a source at any of four wavelengths.

There is thus described a technique which allows manufacture of a multi-contact, single longitudinal mode laser operating CW, and bit rates up to approximately lOGb/s, to be produced from a previously multi-mode Fabry-Perot ridge- waveguide device. The technique embodying the invention is applicable to many types of laser device, and the examples described above should only be considered as such.

The illustrated embodiments of the invention have been used to demonstrate a two-contact operating regime in a high speed, single-mode device and applied it to NRZ modulation schemes. A -3dB bandwidth of 15GHz is found for the two-contact device. For a peak current swing of only 30mA, an extinction ratio of 6dB is found at lOGb/s, indicating a 25% reduction in current swing required. It has also been shown that introducing a reflector structure grating into one of the sections can be used to create high-speed single-mode lasers, without perturbation of the device dynamics.

This simple and repeatable post-processing technique could readily be implemented using any suitable process: for example but not limited to, RIE, IBE, CAIBE or wet chemical etching. Hence promising low-cost devices can be provided for data communications applications. Embodiments of the invention can provide potentially new low drive current operating regimes for future low-cost uncooled single-mode high speed transceivers.

It will be readily appreciated that a device in accordance with the invention can be provided with more than two contact regions . For example a third contact region could be provided that is supplied with an additional control signal. Alternatively, the additional contact region could be electrically^' earthed or operated at a fixed bias. If the earthed region is arranged between the first and second regions, then these regions can be electrically isolated from one another.

Claims

1. A method of modulating a light output of a laser diode which has an active region for producing a light output, and first and second contact regions for supplying respective bias signals to the active region, the method comprising: supplying a first bias current signal to the first contact region; supplying a second bias current' signal to the second contact region; and supplying an RF modulating current signal to the first contact region, wherein each of the first and second bias current signals serves to forward bias at least part of the active region of the laser diode.

2. A method as claimed in claim 1, wherein the first bias current signal is a DC current signal.

3. A method as claimed in claim 1 or 2, wherein the second bias current signal is a DC current signal.

4. A method as claimed in claim 1, 2 or 3, wherein the laser diode includes a third contact region, and the method includes supplying a third current signal to the third contact region of the laser diode.

5. A method as claimed in claim 1, 2, 3, or 4, wherein a modulating current signal is applied to the second contact region.

6. A method as claimed in any one of claims 1 to 4, wherein the laser diode includes at least one further contact region.

7. A method as claimed in claim 5, wherein at least one of the further contact regions is electrically earthed.

8. An optical device comprising: a laser diode having an active region for producing a light output and first and second contact regions for conducting respective bias signals to the active region; a first bias signal unit for supplying a first bias signal to the laser diode via the first contact region thereof, which first bias signal forward biases the laser diode when the device is in use; a second bias signal unit for supplying a second bias signal to the laser diode via the second contact region thereof, which second bias signal forward biases the laser diode when the device is in use; a modulator unit for supplying a modulating bias signal to the laser diode via the first contact region thereof.

9. An optical device as claimed in claim 6, wherein the laser diode includes a ridge-waveguide structure.

10. An optical device as claimed in claim 6 or 7, wherein the laser diode includes a wavelength selective reflector structure.

11. An optical device as claimed in claim 6 or 7 , wherein the first contact region is incorporated into a buried heterostructure device of the laser diode.

12. An optical device as claimed, in claim 6, 7, 8 or 9, wherein the second contact region is incorporated into a buried heterostructure device of the laser diode.

13. An optical device as claimed in any one of claims 6 to 10, wherein the laser diode includes a third contact region.

14. An optical device as claimed in any one of claim 6 to 11, comprising a second modulator unit for supplying a modulating bias signal to the second contact region.

15. An optical device as claimed in any one of claims 8 to 14, comprising at least one further contact region.

16. An optical device as claimed in claim 15, wherein at least one of the further contact regions is electrically earthed.

17. A method of modulating a light output of a laser diode substantially as hereinbefore described with reference to the accompanying drawings .

18. An optical device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.