DE9422388U1 - Optical message transmission with a surface-emitting vertical cavity laser in multi-transverse mode operation - Google Patents

Description

SCHOPPE, ZIMMERMANN;:STG>CKELER:& 0NJKLER

Patent Attorneys ■ PO Box 710867 · 81458 Munich

Agilent Technologies, Inc. (n.d. Corporation of the State of Delaware) 395 Page Mill Road Palo Alto, CA 94306-20.24 U.S.A.

PATENT ATTORNEYS

European Patent Attorneys European Trademark Attorneys

Fritz Schoppe, Dipl.-Ing.
Tankred Zimmermann, Dipl.-Ing. Ferdinand Stöckeier, Dipl.-Ing.
Franz Zinkler, Dipl.-Ing.

Telephone/Telephone 089/790445-0 Fax/Facsimile 089/790 22 1 5 Fax/Facsimile 089/74996977

e-mail: szsz_iplaw@t-online.de

Optical communication with a surface

emitting vertical cavity laser in

Multi-transverse mode operation

Postal address: P.O. Box 710867, 81458 Munich

Office address: Irmgardstraße 22, 81479 Munich

Bankv^*6jneTung/Banker*»: J^poVefeiw-sTsanVprüjWaFd). KoätoiiusÄmer 2 960155028 (BLZ 700202 70) : : PiiMühktä^i315f2§8|3(BLZ00

VAT ID Kir-'/tfATReärsträfflon "number* DB130575439

Optical communication with a surface-emitting vertical cavity laser in multi-transverse mode operation

Description

The present invention relates generally to optical signal transmission and, more particularly, to an optical communication network comprising a multimode optical fiber receiving a multimode light beam from a surface emitting vertical cavity laser operating in multimode or multifilamentary mode.

Optical communication systems are used to transmit information from one place to another. One of the advantages of optical systems is that they have extremely wide bandwidths. This means that optical systems can transmit more information than other types of communication systems, such as radio or microwave technology. For example, almost all long-distance telephone calls are transmitted by optical communication systems because a single optical fiber can carry thousands of calls at the same time. Optical systems also offer the potential to transmit large amounts of digital data for high-speed computers more efficiently and economically than other communication systems.

Every optical communication system comprises at least three elements: a transmitter that transmits a light beam

and modulates the beam with data to be transmitted, a receiver that receives the light beam and recovers the data therefrom, and a medium, such as an optical fiber, that transmits the light beam from the transmitter to the receiver. Typically, the transmitter uses a laser or light-emitting diode ("LED") to generate the light beam. The receiver uses photodetectors or the like to receive the beam. The medium may be an optical waveguide or the like instead of the optical fiber.

Light can travel through an optical medium as a single mode or as multiple modes. In general, a "mode" of an electromagnetic wave can be defined as a stationary pattern of the wave. In the special case of a beam of light (which can be considered an electromagnetic wave in the optical part of the spectrum), a mode is a wave pattern that does not change the shape of its transverse field distribution as it propagates through the medium.

A given optical medium may be able to support many modes or only a single mode. This is determined by the physical parameters such as -- in the case of an optical fiber -- the diameter of the fiber and the difference between the refractive indices of the core and the cladding.

Similarly, many lasers can be made to operate in a single mode or in multiple modes. This can be done by a suitable choice of device structure and driving conditions. Multimode operation has generally been understood to mean that multiple modes exist in a laser cavity. However, studies have shown that multinode laser operation can result in filament formation due to non-uniform gain or attenuation. This is especially true for lasers with large transverse dimensions compared to the wavelength. The convenience

For convenience, the terms "multiple modes" and "multimode" used herein to describe the operation of a laser include both multiple modes in a single laser cavity and multiple filamentation.

Optical communication systems are subject to various types of losses and limitations. Among these are intermodular dispersion, chromatic dispersion and mode-selective losses. These all cause a deterioration in the signal-to-noise ratio. Therefore, it is desirable to eliminate or minimize them as much as possible.

Intermodal dispersion becomes worse as the length of the fiber increases. Intermodal dispersion affects only multimode fibers. Therefore, single-mode fibers are preferred for long-distance communication. The term "long" distance used herein means a distance greater than a few hundred meters, and the term "small" distance means one less than a few hundred meters. It should of course be obvious that this is an approximation. Multimode fibers up to a few kilometers in length have been used successfully, but single-mode fiber is usually used when the required fiber length exceeds a few hundred meters.

Chromatic dispersion also becomes more severe as the fiber length increases, but unlike intermodular dispersion, chromatic dispersion affects both single-mode and multimode fibers. The adverse effects of chromatic dispersion can be minimized by using a highly coherent laser, since such a laser produces a light beam of very narrow spectral width. Accordingly, highly coherent lasers have been preferred for most optical communication systems, particularly for long-distance communication.

Of course, single-mode optical fibers can also be used over short distances (less than a few hundred meters), for example to transmit digital data from one computer to another in a local area network, or even to transmit data between points less than a meter apart in a single computer. However, multimode optical fibers are preferred for short distance optical communication systems, since the relative ease of their packaging and alignment makes them considerably cheaper than single-mode fibers.

A disadvantage of multimode optical media has been that they are subject to mode-selective losses. Mode-selective loss can be characterized as a physical condition that affects the optical characteristics of the medium. These losses can include junctions in the medium, power splitters and other devices connected to the medium, and physical defects such as poor quality connections and misalignment of components. Although such physical connections can be reduced by careful design and construction, in practice it is rarely possible to produce a system that is completely free of them. Therefore, all practically feasible multimode optical communication systems are subject to at least some mode-selective losses.

The actual mechanism by which physical discontinuities produce mode-selective losses is now briefly discussed. Interference between different modes in a multimode medium carrying a coherent light beam produces a speckle pattern. Ideally, this speckle pattern would remain stationary, but in practice it moves somewhat in the medium. The movement of the speckle pattern can be induced by physical nudges or other movement of the fiber itself (relatively slow movement).

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or by laser mode partitioning and the like (relatively fast motion). Motion of the speckle pattern in a system with mode-selective losses results in power changes in the received signal. These changes are caused by the mode-selective losses and result in a deterioration of the signal-to-noise ratio. In digital systems, a deterioration of the signal-to-noise ratio manifests itself as an increased bit error rate.

Mode-selective losses are described in more detail in papers such as Epsworth, R.E., "The Phenomenon of Modal Noise in Analogue and Digital Optical Fibre Systems", Proceedings of the 4th European Conference on Optical Communications, Genoa, September, 1978, pages 492 - 501, and in Kanada, T., "Evaluation of Modal Noise in Multimode Fiber-Optic Systems", IEEE Journal of Lightwave Technology, 1984, LT-2, pages 11 - 18.

Mode-selective losses can be avoided by using a relatively low-coherence light source, such as an LED or a self-pulsing laser diode ("SPLD"), instead of a high-coherence laser. The use of LEDs in optical communication systems is described in Soderstrom, R., et al., "Low Cost High Performance Components of Computer Optical Date Links", Proceedings of the IEEE Laser and Electrooptics Society Meeting, Orlando, Florida, 1989. A disadvantage of using LEDs in optical communication systems is that the coupling efficiency between an LED and an optical fiber is very low. In addition, LEDs are inherently slow, which limits the maximum transmission speed.

SPLDs have been used in such systems as the Hewlett-Packard HOLC-0266 Mbaud Fiber Channel multimode fiber data link manufactured by the assignee of the same. This is in Bates, R.J.S., Multimode Waveguide Computer Data

Links with Self-Pulsating Laser Diodes", Proceedings of the International Topical Meeting on Optical Computing, Kobe, Japan, April, 1990, pages 89 - 90. The coupling efficiency between an SPLD and an optical fiber is better than that between an LED and an optical fiber, but is not yet optimal. In addition, the maximum transmission speed that can be achieved with an SPLD is limited. Neither SPLD nor LED systems have been able to achieve reliable transmission speeds in the range of 1 gigabit per second.

The object of the present invention is to provide an optical communication system operating over short distances in order to transmit data at transmission speeds greater than 1 gigabit per second in a reliable and economical manner.

This object is achieved by an optical communication network according to claim 1.

The present invention provides an optical communication system that can transmit data reliably and economically using multimode optical media at any speed up to and beyond 1 gigabit per second.

Briefly and generally speaking, the invention is embodied in an optical communication system having a vertical cavity surface-emitting laser ("SEL"). A multimode optical medium, such as an optical fiber, is coupled to the SEL. A power supply provides a bias current that drives the SEL into multi-transverse mode operation, preferably with more than two different modes. The SEL produces a light beam that has less coherence than that provided by a single mode laser. This light-

beam is modulated with data carried by an incoming signal. The SEL preferably has an aperture larger than about 8 micrometers ("μιη") through which the modulated light beam is emitted.

The optical medium transmits the modulated light beam from the SEL to a receiver at a remote location. The receiver, which may be closer than 1 meter or further than 100 meters away, recovers the data from the light beam.

A preferred embodiment of the present invention is explained in more detail below with reference to the accompanying drawings.

Fig. 1 is a schematic diagram of a preferred embodiment of an optical communication system according to the invention, and

Fig. 2 is a cross-sectional view of a surface emitting vertical cavity laser of the type used in the communication system shown in Fig. 1.

As shown in the drawings for purposes of illustration, the invention is embodied in a novel optical communication system having a surface emitting vertical cavity laser ("SEL") controlled in multi-transverse mode operation to provide a light beam that reliably and efficiently transmits data over a multi-mode optical medium. To avoid the expense of single-mode fiber for communication over distances of less than a few hundred meters, existing optical communication systems used multi-mode fiber, but such systems were subject to unacceptably high mode selection losses, or used low coherence light sources such as LEDs and SPLDs that were unable to provide sufficiently high

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to achieve transfer speeds.

A communications system according to the invention uses an SEL in multi-transverse mode operation. The SEL provides a light beam that has lower coherence than the high coherence light beams typically used in single mode systems, but higher coherence than the low coherence beams provided by LEDs and self-pulsing lasers. A multimode optical medium transmits the beam from the SEL to a receiver that may be less than 1 meter or 100 meters or more away. The system is capable of transmitting data at any speed up to and above 1.5 gigabits per second with a negligible bit error rate. The system provides all of the advantages, such as ease of alignment, simple packaging and low cost, typically associated with multimode optical media.

A preferred embodiment of the invention is discussed in more detail below. Referring to Figure 1, the invention is embodied in an optical communication network comprising an SEL 11, a power supply 13 which provides a bias current to drive the SEL into multi-transverse mode operation, and a multi-mode optical medium 15 optically coupled to the SEL for transmitting the optical signal from the SEL to a remotely located receiver 17. The SEL is responsive to a signal carrying data (generally designated 19) for providing an optical signal modulated with the data. The receiver 17, optically coupled to the optical medium 15, receives the modulated optical signal and recovers the data (generally designated 21) therefrom.

Various types of multimode optical media, such as optical fibers and waveguides, can be used as the medium 15. The SEL 11 and the receiver 17 are connected by

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suitable couplings 23 and 25 to the medium 15. As discussed in more detail below, the SEL 11 is preferably operated with more than two different transverse modes. As mentioned above, this may involve multiple filamentation.

A preferred method of fabricating the SEL 11 is shown in Fig. 2. The SEL is grown on an n+ GaAs (gallium arsenide) substrate 31. A lower output mirror, e.g., 18.5 pairs of η-doped GaAs/AlAs (gallium arsenide/aluminum arsenide) quarter-wave layers (generally designated 33 in the drawings), is epitaxially grown on the substrate 31. The interface between the layers is graded using a short period variable duty cycle of an AlAs/GaAs/Al(0.3)Ga(0.7)As superlattice ("SPSL"). The SPSL reduces any heterojunction band discontinuities at the GaAs/AlAs interface. The doping level is 1-10-'-^cm"^ in uniform regions and 3-10 ¹⁸ cm~ ³ in graded regions. For simplicity, only a few of the 18.5 layer pairs are shown in the figure. The reflectivity of the lower mirror 33 is 98.9%.

Next, an optical cavity structure 35 is grown. The cavity structure comprises an n-cladding layer 37, a quantum well 39, and a p-cladding layer 41. The cladding layers 37 and 41 comprise Al(0.3)Ga(0.7)As doped to 1-10^cm~3, reduced to 5-10 ¹ ^Cm" ³ adjacent to the quantum well 39. The quantum well 39 comprises three MQWs of strained In(O.2)Ga(0.8)As (indium gallium arsenide) having a thickness of about 80 Å (Å = Angstroms), with GaAs barriers having a thickness of 100 Å.

Above the quantum well 35 there is a highly reflective upper mirror 43. The reflectivity of the upper mirror is greater than 99.96%. The upper mirror 43 comprises, for example, 15 pairs of GaAs/AlAs quarter-wave layers (generally

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my 45), a phase matching layer 47 and a Au layer 49 (Au = gold). A proton isolation region 51 surrounds the outer boundary of the quarter-wave layers 45. As with the lower mirror 33, only some of the quarter-wave layers 45 are actually shown in Fig. 2. The interfaces between the quarter-wave layers are graded in a manner generally similar to the grading of the interfaces in the lower mirror 33. The doping levels are 1-10-^cirr ³ in uniform regions and 5-10 ¹⁸ cm~ ³ in graded regions.

The phase matching layer 47, which consists of GaAs, compensates for phase delays resulting from the limited penetration of the optical field into the Au layer.

The Au layer 49 is about 2000 thick and is deposited after
MBE growth (MBE = Molecular Beam Epitaxy) of the underlying structure as follows. First, a gold layer of 2000 is deposited on the
GaAs phase matching layer 47 is deposited. A thick (greater than 10 /xm) Au button is then plated on top to serve as a mask for proton isolation. The wafer is then proton implanted. Crystal structure damage resulting from the proton implantation provides current confinement and hence gain guidance. Another thick Au button 53, approximately 300 μιη in diameter, is then plated on top. This button 53 is used for solder/die attachment of the finished device to a heat sink. The wafer is then lapped and polished to a diameter of 125 μιη and an annular electrode 55 is patterned on the base surface. A quarter-wave anti-reflection coating 57 of SiO ₂ (silicon dioxide) is deposited in the open region of the electrode 55.

An optical communication system embodying the principles of the invention was constructed using a relatively large area SEL with an aperture of 25 μηη,

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coupled to an optical fiber. A physical discontinuity was intentionally introduced into the fiber. This discontinuity was a gap of several millimeters. The gap was adjustable to provide between 3 dB and 16 dB of attenuation. The length of fiber between the SEL and the gap was 16 meters. This section of fiber was shaken back and forth to simulate the effect of fiber motion. The bit error rate ("BFR") was measured for gaps of various widths. The measured BFRs were less than 10" ¹¹ for attenuations up to 10 db.

In the tests described herein, a wavelength of about 970 nanometers ("nm") was used. It will be apparent that the principles of the invention are equally applicable to devices operating at other wavelengths and that the physical dimensions will vary accordingly.

In a further test, the performance of the large area SEL was compared to that of a smaller SEL with an aperture of 12 μ΄α . The threshold currents were about 6.5 milliamperes (mA) for the large SEL and 4.2 mA for the smaller one. The threshold voltages were 2.7 and 4.5 volts, respectively. The output power at twice the threshold current was 3.6 milliwatts (mW) for the larger SEL and 2.8 mW for the smaller one. The emission wavelength was about 970 nanometers.

The SELs were directly modulated by a non-return-to-zero (NRZ) signal at one gigabit per second with a maximum amplitude of 2 volts and a pseudorandom bit sequence of 2 ¹⁵ -1 through a bias T. The bias levels were multiples of the respective threshold currents. The SELs were directly coupled to a multimode fiber with a graded index of 50/125. The length of the fiber between the SEL and the gap was 16 m and the gap was set for an attenuation of 10 dB. An optical attenuator was placed between the gap and

• O φ # t ? # · 9 ·&ngr; ? φ

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inserted into the receiver to keep the optical power arriving at the receiver at 6 dB above the receiver sensitivity.

The receiver was a Hewlett-Packard model 83442A receiver modified for multimode use with a 60 micrometer InGaAs detector and a multimode FC/PC input connector. The receiver had a -3 dB bandwidth of 0.9 GHz. The AC-coupled receiver output was amplified to 2.0 volts prior to acquisition. The receiver sensitivity was -23 dBm for a noise-limited receiver BFR of 10" ⁹ .

In this test configuration, the SEL with an aperture of 25 μια was operated for 16 hours without a failure, resulting in a BFR of less than 10"^. In further tests, the length of fiber between the SEL and the gap (the gap was set for 10 dB attenuation) was varied between 6 and 406 meters, and in each such case the BFR was less than 10" ¹¹ . The SEL with an aperture of 12 μια was also able to achieve a BFR of less than 10"-^ with the gap set for about 4 dB attenuation.

A highly controlled SEL with a relatively large surface area ("large surface area" means a surface aperture larger than about 8 micrometers) operates with several higher order transverse modes located at slightly different wavelengths. As the size of the aperture increases, the maximum number of transverse modes that can be obtained also increases. Consequently, an SEL with an aperture of 25 μιη can operate with significantly more transverse modes than an SEL with an aperture of 12 μιη.

As the number of transverse modes increases, the optical bandwidth of the light produced by the laser also increases and the coherence of the light decreases.

Spot visibility measurements have shown that the spot visibility of a large-area SEL is lower than that of smaller SELs.

Despite operating with multiple transverse modes, the large-area SEL operates in a stable single longitudinal mode. Longitudinal mode partition noise, which is a consequence of multiple longitudinal modes, is therefore not a significant problem in large-area SELs.

In one test, an SEL with an aperture of 25 μιη was found to operate with at least six different transverse modes at a drive current of 2.3 times the threshold current. The spectral width was *\ = 0.75 nm. When the drive current was reduced sufficiently to cause the laser to enter single-mode operation, the spectral width ^A \ was < 0.08 nm. This measurement was limited by the resolution of the optical spectrum analyzer used for this test. In contrast, an SEL with an aperture of 12 micrometers was found to operate single-mode at a drive current of 1.5 times the threshold current and with two transverse modes at a drive current of 2.5 times the threshold current.

From the foregoing, it will be apparent that an optical communication system according to the invention is capable of transmitting digital data at speeds up to and exceeding 1.5 gigabits per second with very low bit error rates. The invention also offers the advantages such as ease of alignment, simple packaging and low cost associated with systems using multimode optical media. In addition, SELs are expected to be easier and less expensive to manufacture than other types of lasers.

Claims (7)

1. Optical communication network having the following characteristics:
a surface emitting vertical cavity laser structure ( 11 ) having an aperture larger than 8 micrometers through which an optical signal is emitted;
a power supply ( 13 ) providing a bias current to drive the laser into a multi-transverse mode operation in which the laser is responsive to a signal carrying data ( 19 ) to provide an optical signal modulated with the data and to emit the optical signal through the aperture; and
a multimode optical medium ( 25 ) optically coupled to the laser for transmitting the optical signal from the laser to a remotely positioned receiver ( 17 ). 2. A network according to claim 1, wherein the multimode optical medium comprises an optical fiber. 3. A network according to claim 1, wherein the multimode optical medium ( 25 ) comprises an optical waveguide. 4. The network of claim 1, further comprising a receiver ( 17 ) optically coupled to the optical medium ( 25 ) which receives the modulated optical signal and recovers the data ( 21 ) therefrom. 5. A network according to any preceding claim, wherein the aperture of the laser is determined by an element ( 55 ) through which the modulated optical signal is emitted. 6. A network according to any preceding claim, wherein the multi-transverse mode operation comprises more than two different transverse modes. 7. A network according to any one of claims 1 to 5, wherein the multi-transverse mode operation comprises multiple filamentation.