FR2737354A1 - MQW distributed feedback laser for optical communications - has modulators coupled to optical amplifier and built on same indium phosphide substrate with number of epitaxial semiconductor layers and metal electrodes deposited on structure surface - Google Patents

Abstract

The IC includes a laser (L) with a modulator (M1) coupled to an optical amplifier (A1) and a second modulator (M2). All four components are obtained on the same indium phosphate substrate (10) doped with N+ ions. Identical epitaxial semiconductor layers are grown on the flat surface of the substrate while a Bragg grating (12) is provided for the laser. A metal layer (26) is deposited on the lower surface of the substrate and connected to earth. A similar layer (24) is provided on the upper surface . The top layer is then etched to provide electrodes for the laser, modulators and amplifier. The etching is followed by a proton implantation.

Description

INTEGRATED MONOLITHIC LASER-MODULATOR COMPONENT
AMPLIFIER WITH QUANTUM MULTI-WELL STRUCTURE
DESCRIPTION
TECHNICAL AREA
The present invention relates to a monolithic integrated component with a multiple quantum well structure.

 It applies in particular to the field of optical telecommunications and, in particular, to very high speed links and over very long distances.

 The invention is particularly applicable to the generation of short optical pulses, in particular of the soliton type.

 The invention also finds applications in the field of time-division multiplexing as well as in the field of optical computers.

PRIOR STATE OF THE ART
The growing interest in fiber optic links, at very high speed and over very long distances, for example intercontinental submarine links, has prompted various laboratories to study new pulses of the soliton type.

 The latter have the enormous advantage of not being deformed during their propagation in an optical fiber.

 The use of these pulses requires the production of specific optoelectronic components.

 Thus, the development of a short-duration optical pulse source has been the subject of numerous works.

 The short duration optical pulses (less than 10 ps for example) are very interesting not only in the field of solitons but also in the field of time multiplexing.

 A technique for generating short optical pulses has recently been proposed, having the characteristics required for propagation by solitons.

 On this subject, see document (1) which, like the other documents cited below, is mentioned at the end of this description.

 This known technique uses an electro-absorbing modulator which shapes the optical pulses by modulating the light passing through it.

 This light modulation is obtained by applying a high frequency sinusoidal electrical voltage to the modulator.

 The coding of the optical pulses can also be obtained by means of a second modulator which is controlled by an electrical signal of the slot type and which allows or does not allow these optical pulses to pass.

It is thus possible to obtain a complete optical source constituted by - a laser which emits continuously at a wavelength
around 1.55 μm, - a first modulator which shapes the pulses
and - a second modulator which codes these pulses.

 In order to obtain the greatest possible compactness and to reduce the losses which result from the coupling between the laser and the modulators by means of optical fibers, the ideal would be the monolithic integration of this laser and of the modulators.

 The major technological problem of this type of integration is the fact of producing, on the same substrate, optical components requiring active layers whose band gap energies are different.

 Indeed, the laser emits at an energy substantially equal to that of the forbidden band of its active layer.

 On the other hand, the modulator only works at wavelengths shifted towards the red relative to the absorption edge.

 The integration path adopted must meet these two requirements.

 In addition, it must be reliable and remain simple to develop for better performance and better performance.

 Document (2) also discloses an integrated photonic circuit comprising the components described above, namely the laser and the two modulators.

 The integration principle discussed in this document (2) is based on the use of two superposed active layers, one for the laser and the other for the modulators.

 In this case, the light is vertically coupled from the laser to the modulators.

 Other integration channels have been studied, but only for the integration of a laser with a single modulator.

There are two main trends
10 the use of two epitaxies at the rate of one
active layer epitaxy
2 "a" planar "solution which is based on the
local variation in growth rate
of materials between nitride masks or
silica masks whose configuration is
optimized, and which is more and more adopted
for laser-modulator integration.

 Also known from document (3), to which reference will be made, a device which comprises two modulators as well as an amplifier placed between these two modulators and in which the modulators and the amplifier use a single active layer.

 The integrated amplifier between the modulators limits the crosstalk between these two modulators.

 The known techniques mentioned above have drawbacks.

 The techniques which require two epitaxies have the drawback of being complex and difficult to implement from the point of view of the epitaxy and of the technological process (epitaxy and technological steps on a non-planar substrate).

 It is therefore difficult to maintain the performance of the components during their integration.

 The "planar" techniques on the other hand have the advantage of using a process similar to that which is it for discrete components and therefore simpler to implement with better efficiency.

 In the case of locally variable growth rate epitaxy, an optimization of the mask configuration is necessary.

 In addition, surface preparation is a critical step to perform.

 Therefore, it is a delicate process to implement.

 In addition, the change in wavelength obtained with this technique is gradual and extends over a large transition zone, capable of inducing disturbances.

 As regards the lasertandem structure of modulators, which is known from document (2), the spatial separation of the two modulators is very low (50 μm).

 This proximity is likely to induce effects of crosstalk between the two channels which polarize the diodes used.

 This phenomenon would be all the stronger as the speed increases and this would reduce the transmission performance of the integrated component.

 This drawback could be remedied by using greater spatial separation, but this would increase the insertion losses.

 Also known from document (4), to which reference will be made, is a monolithic integrated laser-modulator component with a quantum multi-well structure.

 What this document (4) discloses is found in document (5) to which we will also refer.

 This same monolithic integrated laser-modulator component is also mentioned in document (6).

 Document (7) also discloses a component integrated with two tandem modulators.

STATEMENT OF THE INVENTION
The present invention relates to an integrated monolithic electro-optical component comprising - a distributed feedback laser, ("distributed
laser feedback ") - two electro-absorbing modulators with Stark effect
confined and - an optical amplifier placed between the two
modulators, component which solves the problems previously mentioned.

 The manufacture of this component is very simple and very suitable for industrial production.

 In addition, this component has a very high reliability and has better performance than the known components, previously mentioned.

 In particular, the component known from document (3) requires, for its use, to be coupled to a laser via an optical fiber.

 This coupling induces optical losses and a stability problem because the optical coupling is liable to change over time.

 The present invention solves these problems by using the same active layer produced by means of a single plane epitaxy to form the laser, the two modulators and the optical amplifier placed between them.

Specifically, the subject of the present invention is an integrated monolithic electro-optical component, characterized in that it comprises - a monocrystalline substrate, - a laser comprising a stack of layers
semiconductors epitaxially grown on said substrate from
which an active layer and a Bragg grating
periodic setting the emission wavelength of the
laser at a value slightly higher than the
wavelength corresponding to the maximum of the peak
gain of the laser, - first and second electro-optical modulators
each formed from a stack of layers
semiconductors epitaxially grown on said substrate from
which an active absorbent layer, - an optical amplifier placed between these first and
second modulators, this amplifier being formed
a stack of semiconductor layers
epitaxially grown on said substrate including a
active layer, - means for injecting layers onto the stack
from the laser a current causing the emission by the
active layer of the radiation laser crossing the
absorbent layer of the first modulator in the plane
of this layer, - means for applying to the stack of layers
of the first modulator a reverse electrical voltage
causing absorption of said radiation by the
absorbent layer of this first modulator, - means for injecting layers onto the stack
from the optical amplifier a current causing
amplification of said radiation by the active layer
of this amplifier, and - means for applying to the stack of layers
of the second modulator a reverse electrical voltage
causing absorption of said radiation by the
absorbent layer of this second modulator, and in that the active layer of the laser, the active layer of the optical amplifier and the respective absorbent layers of the first and second modulators are formed by the same epitaxial structure with quantum multi-wells, each of the first and second modulators operating according to the effect
Stark confined.

 The integrated optical amplifier, placed between the first and second modulators, reduces crosstalk and compensates for optical losses.

 The result is a much more efficient circuit than that mentioned in document (2).

 According to a preferred embodiment of the optical component object of the invention, the substrate is made of InP.

 The component is then capable of generating and coding short optical pulses at 1.55 pin, for example of the soliton type.

 The quantum wells of the epitaxial structure can be constrained ("strained").

Each active layer and each absorbent layer can be formed of quantum wells in
InGaAsP or in undoped InGaAs, these quantum wells being separated by barrier layers also in
InGaAsP or undoped InGaAs, the composition of In and
P being chosen so that the quantum wells have a band of prohibited energy lower than that of the barrier layers.

BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood on
reading the description of examples of
realization given below, purely
indicative and in no way limiting, by making
reference to the attached single figure which is a
schematic cross-sectional view of a mode
of the component
monolithic integrated object of this
invention, comprising a laser, two modulators
and an optical amplifier between them.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS
The integrated monolithic electro-optical component, which is schematically represented in this figure, successively comprises - a laser L, - a first modulator M1, - an optical amplifier Al and - a second modulator M2 which are produced on the same substrate 10 with a surface 16 3 plane, in N + doped InP, at 5x10'8 ions / cm3.

 On this substrate 10, are identical epitaxial semiconductor layers for the laser L, the first modulator M1, the optical amplifier Al and the second modulator M2, except for the presence of a periodic Bragg grating 12 for the laser .

 The semiconductor layers are epitaxied according to the chemical vapor deposition technique using organometallics, known by the abbreviation MOCVD.

There is, in order - a buffer layer 14 made of N doped InP, typically at
2 X 1018 ions / cm3, 500 nm thick, - a lower optical confinement layer 16 in
Undoped InGaAsP 100 nm thick, - an active layer 18 150 nm thick,
comprising from 5 to 12 quantum wells 18a separated by
potential barriers 18b, realized in InGaAsP
undoped, - an upper optical confinement layer 20
also in undoped 100 nm InGaAsP
thick, - the Bragg 12 network which is kept only from
side of the laser L, - a layer 22 of P doped InP thus completing the
PIN diodes of laser L, modulator M1,
the optical amplifier A1 and the modulator M2 and
ensuring the burial of the active structure, this 16 3
layer 22 being doped with 10l8 ions / cm3 and having a
thickness of 1.8 pins, and - an electrical contact layer 24 made of P + doped InGaAs
at 1019 ions / cm3, 200 nm thick.

 The quantum wells 18a can be constrained.

 The composition in In and in P of the confinement layers 16 and 20 must be such that the optical index of these layers is greater than that of the layers 14 and 22 of the diodes but less than that of the quantum wells 18a.

 In addition, the composition in In and P of the barrier layers 18b of the multi-quantum well structure must be such that the forbidden energy band of these barriers is greater than that of the quantum wells 18a.

As an example - layers 16 and 20 are made of a material
quaternary InGaAsP whose wavelength
photoluminescence is around 1.2 pins, - the barrier layers 18b are made of a material
quaternary InGaAsP whose wavelength
photoluminescence is around 1.25 pin, and - the quantum wells 18a are made of a material
quaternary InGaAsP whose wavelength
photoluminescence is around 1.55 pins.

 The number of quantum wells is for example equal to 10 in order to improve the modulation performance without reducing that of the laser.

 The thickness of these ten wells is typically 10 nm and that of the corresponding barriers 5 nm.

 The periodic Bragg grating 12 consists of an undoped InP / InGaAsP / InP three-layer material, etched to form a periodic structure whose pitch p fixes the emission wavelength of the laser.

 For layers 12a, 12b and 12c respectively 50, 30 and 30 nm thick, a pitch p of 240 nm is used so as to obtain a laser operating at 1.550 pin.

 The Bragg grating is produced by conventional holography and then wet chemical attack techniques, using an appropriate mask on the part corresponding to the modulators and the optical amplifier in order to form this grating only on the laser part.

 Layers 12, 14, 16, 18 and 20 are formed during a first planar epitaxy and layers 22 and 24 are formed during a second planar epitaxy.

 To make the component shown in the figure, different integration techniques can be used, in particular the "ridge-ridge" technique or the "BRS-ridge" technique.

 The component of the figure further comprises a metal layer 26 formed on the lower surface of the substrate 10 and intended to be brought to ground potential.

 This component also includes an upper metallic layer deposited on the contact layer 24.

In order to ensure the electrical insulation of the laser L, of the modulator M1, of the optical amplifier
A1 and the modulator M2, an etching of this upper metal layer and of the contact layer 24 is carried out.

 There is thus formed - an upper electrode 28 on the laser L, - an upper electrode 30 on the modulator M1, - an upper electrode 32 on the amplifier A1 and - an upper electrode 34 on the modulator M2.

 This etching is followed by implantation of protons in the trenches 36, 38, 40 thus formed and over the entire thickness of the layer 22.

 The upper electrode 28 of the laser L is connected to a current source I1.

 The electrode 30 of the modulator M1 is reverse biased by a voltage source Vil.

The optical amplifier electrode 32
Al is connected to a current source I2.

 The upper electrode 34 of the modulator M2 is reverse biased by a voltage source Vi2.

 It can be seen that the successive epitaxy of the different layers leads to excellent optical coupling between the laser, the first modulator, the optical amplifier and the second modulator.

 The length LL of the laser ribbon is typically of the order of 300 pins to 400 pins.

 The length LA1 of the amplifier ribbon is typically 500 pins.

 The length LM1 of the ribbon of the modulator M1 and the length LM2 of the ribbon of the modulator M2 are typically 200 pin.

 In order to minimize the capacity of the component of FIG. 1, the electrical contact sockets, on the side of the modulator M1, of the amplifier Al and of the modulator M2 are obtained using studs produced on polyimide.

 It is specified that the end of the component of FIG. 1, which is situated on the side of the laser L, is coated with a set of highly reflective layers C1, for example made of Al203 and Si.

 The other end of this component, through which the light comes out and which is located on the side of the modulator M2, is coated with an anti-reflection layer C2, for example made of SiOx.

 The component shown in the figure makes it possible to constitute an integrated source for the generation and coding of short optical pulses, in particular of the soliton type.

 The laser L then operates continuously.

 The M1 modulator is used for shaping and generating short optical pulses and operates at high frequency.

 The M2 modulator is intended to code these pulses and operates at high frequency.

 The modulators M1 and M2 should be moved away from each other to avoid electrical interference between these modulators.

 The amplifier A1 operates continuously and makes it possible to amplify the light coming from the modulator M1 to avoid optical losses due to the remoteness of the modulators M1 and M2.

 An important characteristic of the invention is the wavelength compatibility of the elements integrated using a positive shift of the emission wavelength of the laser towards the long wavelengths thanks to the Bragg grating. .

 The large bandwidth of the modulators and the low electrical crosstalk between them (which is made possible by the integration of the amplifier between the two modulators) allow the generation and coding of short optical pulses, of the soliton or other type. .

 The optical output power of a component according to the invention is high thanks to the optical amplifier integrated between the modulators, which makes it possible to reduce the noise linked to the amplification necessary before sending, in an optical fiber, a pulse of too low energy.

 The integration of the optical amplifier makes it possible to reduce the crosstalk between the adjacent modulators to less than 40 dB and the propagation losses in the component.

 These losses can be high as reported in document (7) (of the order of 15 dB).

 The amplifier has a gain of 8 dB for a current of 200 mA.

 The modulators make it possible to obtain an extinction rate better than 25 dB for an applied voltage of less than 2V.

 This component works successfully at 20 Gbit / s.

 The documents cited in the present description are the following (1) M. Suzuki, H. Tanaka, K. Utaka, N. Edagawa and Y.

Matsushima, "Transform-limited 14 ps optical pulse
generation with 15 GHz repetition rate by InGaAsP
electroabsorption modulator ", Electronics Letter,
1992, vol.28, n011, pp 1007-1008 (2) K. Sato, K. Wakita, I. Kotaka, Y. Kondo and M.

Yamamoto, "Multisection electroabsorption
modulators integrated with distributed feedback
lasers for pulse generation coded at 10 Gbit / s ",
Electronics Letters, 1994, vol.30, n014, pp 1144
1145 (3) N. Souli, F. Devaux, A. Ramdane, P. Krauz, A.

Ougazzaden, F. Huet and M. Carré, "10 Gbit / s high
performance MQW tandem modulator for soliton
generation and coding ", Electronics Letters, 1994,
vol.30, n020, pp 1706-1707 (4) A. Ramdane, F. Devaux, A. Ougazzaden, Component
integrated monolithic laser-modulator with structure
multi-quantum wells, French patent application
ne9306565 of 2 June 1993 (5) EP-A-0627798 (6) A. Ramdane, A. Ougazzaden, F. Devaux, F. Delorme,
M. Schneider, J. Landreau and A. Gloukhian, "Novel
high performance strained layer MQW monolithically
integrated DFB laser-electroabsorption modulator
using one identical single active layer ", 14th
IEEE International Semiconductor Laser Conference,
September 19-23, 1994, Hawaii, paper n M4.6 (7) H. Tanaka, S. Takagi, M. Suzuki and Y. Matsushima,
"Optical short pulse generation and data
modulation by a single-chip InGaAsP Tandem
integrated electroabsorption modulator (TEAM) ",
Electronics Letters, 1993, vol.29, ne11, pp 1002
1004.

Claims (4)

 1. Integrated monolithic electro-optical component, characterized in that it comprises - a monocrystalline substrate (10), - a laser (L) comprising a stack of layers  semiconductors epitaxially grown on said substrate from  which an active layer and a Bragg grating  periodic setting the emission wavelength of the  laser at a value slightly higher than the  wavelength corresponding to the maximum of the peak  gain of the laser, - first and second electro-optical modulators  (M1, M2) each formed by a stack of layers  semiconductors epitaxially grown on said substrate from  which an active absorbent layer, - an optical amplifier (Al) placed between these first  and second modulators, this amplifier being  formed of a stack of semiconductor layers  epitaxially grown on said substrate including a  active layer, - means (I1) for injecting onto the stack of  layers of the laser a current causing emission by  the active layer of the laser through a radiation  the absorbent layer of the first modulator in the  plane of this layer, - means (Vil) for applying to the stack of  layers of the first modulator an electrical voltage  reverse causing absorption of said radiation  by the absorbent layer of this first modulator, - means (I2) for injecting on the stack of  layers of the optical amplifier a current  resulting in amplification of said radiation by the  active layer of this amplifier, and - means (Vi2) for applying to the stack of  layers of the second modulator an electrical voltage  reverse causing absorption of said radiation  by the absorbent layer of this second modulator, and in that the active layer of the laser, the active layer of the optical amplifier and the respective absorbent layers of the first and second modulators are formed by the same epitaxial structure with quantum multi-wells ( 18), each of the first and second modulators operating according to the confined Stark effect.  2. Optical component according to claim 1, characterized in that the substrate (10) is made of InP.  3. Component according to claim 2, characterized in that the quantum wells (18a) are constrained.  4. Component according to any one of claims 2 and 3, characterized in that each active layer and each absorbent layer are formed of quantum wells (18a) in InGaAsP or in undoped InGaAs, these quantum wells being separated by layers- barriers (18b) also in InGaAsP or in Undoped InGaAs, the composition in In and P being chosen so that the quantum wells have a band of prohibited energy lower than that of the barrier layers.