WO2003079507A2

WO2003079507A2 - Directly modulated laser diode with chirp control

Info

Publication number: WO2003079507A2
Application number: PCT/GB2003/001185
Authority: WO
Inventors: Hiroshi Nakamura; Yee Loy Lam; Yuen Chuen Chan
Original assignee: Denselight Semiconductors Pte Ltd.; Finnie, Peter, John
Priority date: 2002-03-15
Filing date: 2003-03-17
Publication date: 2003-09-25
Also published as: WO2003079507A3; AU2003216843A1; GB0206204D0

Abstract

A semiconductor laser with high-speed direct modulation comprises a gain region (A) in which optical radiation is amplified, a phase region (B), having a higher energy band gap than the gain region and thereby substantially transparent to the optical radiation, means for applying an RF modulation signal to the gain region, and means for applying an RF modulation signal to the phase region, wherein the signal applied to the phase region causes a phase modulation which compensates for wavelength variations in the optical radiation caused by the direct modulation of the gain region.

Description

DIRECTLY MODULATED LASER DIODE WITH CHIRP CONTROL

Field of the Invention

The present invention relates to the direct modulation of a laser diode for use in optical communication systems and, in particular, to a directly modulated laser diode source with integral control of the level of optical signal chirp.

Background to the Invention

The distributed feedback (DFB) laser is widely used for optical fiber communication because of its superior optical performance, such as single mode operation, large side-mode suppression ratio, high power and temperature stability. Direct modulation through the variation of the laser diode's injection current is one of the means whereby electronic data is transferred onto the optical carrier. For a transmission speed of 2.5 Gb/s (OC48 communication standard), it is common to implement direct modulation as it is of low cost in associated electronics and does not have any serious technical problems.

For higher speeds of direct modulation, the phenomenon of chirp becomes significant. This is due to the dependence of the material refractive index on the carrier density. As the injected current increases, the carrier density in the laser diode increases, lowering the refractive index and leading to a wavelength chirp. The chirping effect causes a broadening of the light spectrum. Together with the dispersion of the transmitting optical fiber, this leads to bit errors in the case of long distance transmission. Consequently, direct modulation of DFB lasers at 10 Gb/s is useful only for short distance (up to a maximum of 10 km) transmission. In order to transmit data at 10 Gb/s or at even higher speeds over long distances of from 10 km up to thousands of km, external modulation of the light signal is commonly used because of the lower chirp imparted. There are two kinds of optical modulators currently being used: the Mach-Zehnder interferometer (MZI) type and the electro-absorption (EA) type. Presently, the EA modulator is sometimes fabricated directly adjacent the DFB laser diode in a monolithic chip. The fabrication process of such integrated EA-DFB laser is extremely complicated.

Although external modulation of the light source can reduce the associated dispersion penalty, it is significantly more complex as compared to a directly modulated laser diode. There is therefore a strong motivation to develop a directly modulated laser diode with low chirp performance.

One approach to a directly modulated source with reduced chirp is through push-pull modulation. Here, the laser diode is operated such that the mean carrier density has a constant value. This should mean that the refractive index variation is minimal, implying low chirp modulation. Push-pull modulation is normally implemented in a three-contact laser, where the end contacts are modulated in- phase and the centre contact has a compensation modulation applied in anti-phase such that variations in the mean carrier density are reduced. In another configuration, the centre contact of the push-pull modulated laser is biased with a constant current, and the end contacts are modulated in anti-phase so that the current to one end contact is increased while the current to the other end contact is decreased by an exactly equal amount. In this case, the symmetry of the modulation leads to the increase in electron density at one end being automatically compensated by changes in the electron density at the other end, irrespective of the bias or modulation level.

Several such multi-section laser devices have been proposed in the literature. These include devices whose various sections have different band gap energies and which are selected such that the device lasing wavelength is located in the overlap region of the gain spectra of the sections. By selecting the lasing wavelength to be in the overlap region of the gain spectra of the sections, use can made of the positive detuning in one section and negative detuning in the other section, by compensating for positive chirp in one with negative chirp in the other. However, this means that if any section is not deliberately pumped above transparency, it will be absorbing to the emitted light. Moreover, as the gain spectrum is not symmetric, the achievable wavelength range of zero chirp is quite limited.

In other devices, a current or voltage phase shifter is utilised to balance refractive index variations for wavelength stabilisation. However, it should be noted that current phase shifter concepts only work well with single frequency sinusoidal signals. This leads to a problem when operating with square wave signals, as it is important to ensure that all frequency components of the signal are phase-shifted together in the two sections. Furthermore, the phase shifting section often comprises the same active layer as the gain section with the same band gap energy. Therefore, as the phase shifting section is kept below lasing threshold, it will be absorbing at the wavelength of the light emitted by the gain section and consequently the optical loss of the device will be high.

There is thus a need for a compact laser diode source for direct modulated at the 10 Gb/s level, which is characterised by low loss and the ability to control and correct for optical chirp over a wide bandwidth without the need for overly complex drive electronics. Summary of the invention

According to a first aspect of the present invention a modulated semiconductor laser comprises: a gain region, in which optical radiation is amplified; a phase region, having a higher energy band gap than the gain region and thereby substantially transparent to the optical radiation; means for applying an RF modulation signal to the gain region; and, means for applying an RF modulation signal to the phase region, wherein the signal applied to the phase region causes a phase modulation which compensates for wavelength variations in the optical radiation caused by the direct modulation of the gain region.

The higher energy band gap of the phase region ensures that optical loss in this region is automatically low at the lasing wavelength without the need to actively drive the region to achieve transparency. The higher energy band gap of this region may be obtained by a number of techniques, including growth and regrowth, selective area epitaxy and quantum well intermixing (QWI).

Preferably, the laser comprises means for deriving the modulation signal to the gain region and the modulation signal to the phase region from a common RF modulation signal. The laser may comprise means to time differentiate the RF modulation signal to the gain region in order to derive the RF modulation signal to the phase region. The laser may further comprise means to invert the RF signal to the gain region in order to derive the RF signal to the phase region.

As the chirp in the laser output is proportional to the change of frequency with time, by supplying an RF signal and its time derivative to the two sections of the device, it is possible to compensate for the refractive index variations in one section with the other. This is particularly important when operating with square wave signals, as it is important to ensure that all frequency components of the signal are phase-shifted together in the two sections. Preferably, the phase region includes a Distributed Bragg Reflector grating.

A distributed grating structure provides additional wavelength selectivity.

Also provided is a semiconductor laser according to the first aspect of the present invention, which further comprises: a first region having a Distributed Feedback Structure; and a second region having a Distributed Feedback Structure, wherein the gain region and the phase region are located between the first and second regions. The distributed feedback structures can provide alternative or additional feedback to discrete feedback from the device facets.

According to a second aspect of the present invention a method of compensating for chirp in a directly modulated semiconductor laser comprises the steps of: providing a gain region of the semiconductor laser for amplification of optical radiation; providing a phase region having a higher energy band gap than the gain region and thereby substantially transparent to the optical radiation; applying an amplitude modulated RF signal to the gain region; and applying an amplitude modulated RF signal to the phase region, wherein the signal applied to the phase region causes a phase modulation of the light which compensates for wavelength variations in the optical radiation caused by the direct modulation of the gain region. Thus the present invention applies a new chirp compensating technology to realise a zero-chirp or controlled low-chirp laser diode suitable for 10 Gb/s direct modulation. The present invention employs both a new laser diode structure and a new driving configuration, in comparison with prior art sources.

Brief Description of the Drawings

Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a schematic of a two-section diode laser according to the present invention; Figure 2 shows the variation in time of the wavelength shift and photon density associated with the diode laser of Figure 1 ; and,

Figure 3 shows a schematic of a four-section diode laser according to the present invention.

Detailed Description

Figure 1 shows a first embodiment of the proposed low-chirp laser diode, including a schematic of the RF driving arrangement. The laser is constructed from two parts: a gain region (part-A) with an bandgap energy that determines the emission wavelength and which does not contain a grating, and a distributed Bragg reflector (DBR) region (part-B), which has a higher energy bandgap than the gain region and also contains a diffractive grating structure within the waveguide. The higher energy bandgap means that the DBR region is practically transparent to the emitted laser light. The higher bandgap region can be implemented by growth and regrowth, selective area epitaxy or by quantum well intermixing. Here, the gain region is driven by a radio frequency (RF) modulation signal, and the DBR region is driven by a signal comprising a DC component and a component derived from the RF signal. The component synthesized from the RF signal may comprise a direct copy of the RF signal, the time differential of the RF signal, dfdt(RF), the inverse-RF signal, and the time differential of the inverse-RF signal, cf/cff(inverse-RF).

The operation of the proposed laser diode structure of Figure 1 is described as follows. When a time-varying (RF) voltage is applied to the gain region (Part A), the amplitude (or intensity) of the light is modulated. At the same time, the phase of the light is also modulated through an associated refractive index variation. Hence, a variation of the emission wavelength (optical frequency) occurs, which is proportional to the time differential of the phase change resulting from the refractive index variation, as shown in Figure 2. Consequently, by modulating the drive current of the DBR region (Part-B), the emission wavelength of the light can similarly be modulated. When the drive current of the DBR region (Part-B) contains a component related to the time differential of the RF signal, the wavelength shift (chirp) from the gain region can be compensated, thus realising a chirp controllable laser diode. In contrast to prior art systems, this permits compensation of the chirp over all frequency components of more complex (non-sinusoidal) signals, including square wave signals, which are widely used for digital encoding.

Figure 3 shows a second embodiment of a low chirp laser in accordance with the present invention, in which the laser diode comprises four parts (A, B, C and D). Here, Part-A is a gain section with an embedded grating, Part-B is also a gain section but without a grating, Part-C is a region without a grating but with a higher band gap energy as compared to Parts A and B, and finally, Part-D is a further gain section with an embedded grating (similar to Part-A). As shown, Part-B and Part-C are driven by differential RF modulation signals with a DC bias, where the amplitude of the RF signal and the level of DC bias can be adjusted independently. Furthermore, the RF signals to Part-B and Part-C can be adjusted independently by means of the two attenuators (ATT). The end sections of the device, Part-A and Part-D are simply driven by a DC source, in order to provide gain. In the laser structure of Figure 3, the lasing wavelength is mainly determined by Part-A and Part-D, which are gain sections with embedded distributed feedback gratings. RF modulation of the optical signal is mainly performed by Part-B but, as discussed above, unwanted phase modulation occurs in addition to the desired amplitude modulation. However, in this structure, phase modulation resulting in Part-B is compensated by Part-C, where phase modulation occurs without amplitude modulation. This is as a result of the lasing wavelength being far from the band edge of Part-C of the device, as its energy band gap is larger than that of the other three parts. As before, this also ensures that optical loss in the phase section is kept low.

Claims

1. A modulated semiconductor laser comprising:- a gain region, in which optical radiation is amplified; a phase region, having a higher energy band gap than the gain region and thereby substantially transparent to the optical radiation; means for applying an RF modulation signal to the gain region; and, means for applying an RF modulation signal to the phase region, wherein the signal applied to the phase region causes a phase modulation which compensates for wavelength variations in the optical radiation caused by the direct modulation of the gain region.

2. A semiconductor laser according to claim 1, comprising means for deriving the modulation signal to the gain region and the modulation signal to the phase region from a common RF modulation signal.

3. A semiconductor laser according to claim 2, comprising means to time differentiate the RF modulation signal to the gain region to derive the RF modulation signal to the phase region.

4. A semiconductor laser according to claim 2 or 3, comprising means to invert the RF signal to the gain region to derive the RF signal to the phase region.

5. A semiconductor laser according to any one of claims 1 to 4, in which the phase region is quantum well intermixed.

6. A semiconductor laser according to any one of claims 1 to 5, wherein the phase region includes a Distributed Bragg Reflector grating.

7. A semiconductor laser according to any one of claims 1 to 6, including:- a first region having a Distributed Feedback Structure; and a second region having a Distributed Feedback Structure, wherein the gain region and the phase region are located between the first and second regions.

8. A method of compensating for chirp in a directly modulated semiconductor laser comprising the steps of:- providing a gain region of the semiconductor laser for amplification of optical radiation; providing a phase region having a higher energy band gap than the gain region and thereby substantially transparent to the optical radiation; applying an amplitude modulated RF signal to the gain region; and applying an amplitude modulated RF signal to the phase region, wherein the signal applied to the phase region causes a phase modulation of the light which compensates for wavelength variations in the optical radiation caused by the direct modulation of the gain region.