Photonic Integrated Circuits

ample of a switch state is i l lustrated in Figure 2. The design and fabrication process al low typical crosstalk levels less than -40 dB.

When contrasting the systems requirements on switch arrays wi th data for the gate amplif ier switch arrays, the latter clearly represent a very interesting option. It should be noted however, that other alternatives, such as digi tal opti­cal switch arrays, 8 combined w i th laser amplif iers and /o r erbium-doped fiber amplif iers, overcome the inherent loss problems (Eq. [1]) at the expense of a more complex switch element structure, the excess loss of wh ich has to be signif i­cantly lower than 3 dB.

It appears that the switch arrays described here can be developed to sizes required for systems applications ( ≥ 8X8) 4

and references therein, and that, i n general, switch arrays wi th integrated optical amplif iers w i l l br ing the photonic network significantly closer.

A C K N O W L E D G M E N T

This work was performed in part w i th in the R A C E 1033 (OSCAR) project and the R A C E 2028 ( M W T N ) project.

MATS GUSTAVSSON is with Ericsson Telecom, Stockholm, Swe­den; MATS JANSON is with Ericsson Components, Kista, Swe­den; LARS THYLÉN is presently at the Royal Institute of Technology, Stockholm, Sweden.

REFERENCES

1. B. L. Pate l et al., "T ransmiss ion at 10 G b / s e c over 100 k m u s i n g a h i g h per formance e lectroabsorpt ion modu la to r a n d the direct p rech i rp ing technique," Proc. 18th Eu ropean Conf . o n Op t i ca l C o m m u n i c a t i o n , Ber­l i n , 1992, postdeadl ine paper , 859-862.

2. S. Johansson, "Transparent op t ica l mu l t i car r ie r networks , " Proc. 18th European Conf . o n Op t i ca l C o m m u n i c a t i o n , Ber l i n , 1992, 781-786.

3. M . Ikeda et al., "Mono l i t h i c L D opt ica l mat r ix swi tches," Proc. 13th Eu ropean Conf . o n Op t i ca l C o m m u n i c a t i o n , H e l s i n k i , 1987, 227-230.

4. M . Janson et a l . , "Monol i th ίca l ly integrated 2 X 2 I n G a A s P / I n P laser a m ­pl i f ier gate sw i tch arrays," Electr . Lett . 28, 1992, 776-778.

5. D . A . O . Dav ies et al., " Integrated lossless 1 to 4 opt ica l sp l i t t e r / comb ine r operat ing at 1.55 μ m , " O S A Proc. Op t i ca l A m p l i f i e r s a n d Thei r A p p l i c a ­t ions, Santa Fe, N . M . 1992, postdead l ine paper , 43-46.

6. M . G . Y o u n g et al., " A 1X16 photon ic sw i t ch operat ing at 1.55 μ m w a v e ­length based o n opt ica l ampl i f ie rs a n d a pass ive opt ica l spl i t ter," O S A Proc. Op t i ca l A m p l i f i e r s and The i r App l i ca t i ons , Santa Fe, N . M . 1992, postdeadl ine paper , 33-37.

7. M . Gus tavsson et al., "Mono l i t h i ca l l y integrated 4 x 4 I n G a A s P / I n P laser ampl i f ie r gate sw i t ch arrays," O S A Proc. Op t i ca l A m p l i f i e r s and The i r App l i ca t i ons , Santa Fe , N . M . 1992, postdead l ine paper , 38-42.

8. J. F. V inchan t et al., "F i rst po la r i za t ion insensi t ive 4 X 4 sw i t ch mat r i x o n InP w i t h d ig i ta l opt ica l swi tches," Proc. 18th Eu ropean Conf . o n Op t i ca l C o m m u n i c a t i o n , Ber l i n , 1992, 341-344.

F U R T H E R R E A D I N G

M.J . O ' M a h o n y , "Semiconductor laser opt ica l ampl i f ie rs for use i n future f iber systems," J. L igh t . Tech. 6, 1988, 531-544.

L. Thy lén , " G u i d e d - w a v e sw i tch ing devices," Proc. 17th E u r o p e a n Con f . o n Op t i ca l C o m m u n i c a t i o n , Par is , France, 1991, 109-116.

Figure 1. TPON/BPON PIC. Chip dimensions are: length=3.75 mm, width=0.75 mm, and thick­ness=0.12 mm.

Photonic Integrated Circuits for the Access Network

BY P E T E R J . WILL IAMS, P A U L M. C H A R L E S , A N D

A N D R E W C . C A R T E R

Photonic Integrated Circuits (PICs) are single chips of semi­conductor onto wh ich a number of optical and optoelec­tronic subcomponents are monoli thical ly integrated to form, for instance, an optical terminal circuit. The motivat ion is the same as was the case at the dawn of the Si circuit age: the promise of reduced cost and size, increased component rel i­abil i ty, and more practical solutions to systems' require­ments compared to implementat ions based on discrete devices. There is a very w ide range of potential applications for PICs, inc luding many w i th in telecommunications, such as optical access terminals, L A N s , trunk l inks, and intra­exchange l inks. Mi l i ta ry /av ion ic applications are in network­ing , E W / E C M (Electronic War fa re /E lec t ron ic Counter Measures) and sensing. Each application places different priorit ies on the integration; for example, i n domestic or business optical access systems, the priorities are low cost and h igh reliabil ity. A s another example, for avionic and mil i tary applications, functionality, size, and h igh reliabil ity are l ikely to have higher pr ior i ty than absolute cost. To min imize manufacturing requirements, the development of integration processes for fabricating different functionality PICs using common technology and design rules is essential.

In this article we describe the development of a generic PIC process aimed at meeting these requirements. A s an example we describe the development of PICs for Access Network applications, where the ma in dr iv ing force is re­ducing cost and increasing the reliabil i ty of optical modules over those that might be fabricated using discrete compo­nents. The integration technology is sufficiently mature such that PICs may be designed and fabricated to meet the re­quirements of particular systems and a number of different PICs have been prepared for various applications. These include 1300/1530 and 1530/1300 n m bidirectional trans­ceivers, exchange/subscriber PICs for broadband l inks 1 and narrow band add-drop optical mult iplexers. 2 In particular, we describe the development of PICs of the former type, inc luding those developed specifically for the proposed Te­lephony and Broadband access over a Passive Opt ical Net­work system, T P O N / B P O N . 3

P R O C E S S T E C H N O L O G Y , D E S I G N CHIP L A Y O U T ,

A N D S U B C O M P O N E N T C H O I C E

A photograph of our latest T P O N / B P O N PIC is shown in Figure 1 (left). This chip is designed to meet the required functionality of a customer access 1300 n m telephony transmit and 1300 telephony/1530 n m broadband receive system. To meet the wave­length requirements, the circuit is based on the I n P / I n G a A s P material system and consists of a 1300 n m wavelength band laser integrated w i th a 3dB coupler, f rom wh ich the signal is fed into a Mach-Zehnder interferometer acting as a 1300/

16 O P T I C S & P H O T O N I C S N E W S / M A R C H 1993

Figure 2. SEM micrograph showing a typical laser - waveguide interface of a TPON/BPON PIC highlighting the planar laser to waveguide butt join and the integrated, semi-insulating substrate laser process details.

Figure 3 . Network facet, light output power versus drive current for a TPON/BPON PIC.

1530 n m multiplexer. Actua l transmit and receive wave­lengths for the telephony data are set w i th in 30 n m wide bands each side of 1300 nm: 1270-1300 n m transmit and 1310-1340 n m receive. Accurate placement of the transmit wavelength is achieved by the use of single-mode Distr ib­uted Feedback (DFB) lasers. 4 The PIC designed for the s im­pler broadband only bidirectional system is similar to the T P O N / B P O N chip, but lacks the addit ional 3 dB coupler that is used for the 1300 n m band direction mult ip lexing. Other specialized PICs make use of Distr ibuted Bragg Re­flector (DBR) laser structures 2. Both DFB and D B R lasers are particularly flexible when used in integrated circuits and have the further benefit of being less sensitive to wavelength variations wi th temperature than Fabry-Perot laser struc­tures. The above wavelength allocation al lows optical filters to be included in the module to reduce transmit/receive optical crosstalk. Accurate wavelength allocation is also es­sential for optical modules work ing in mult i -wavelength, multi-service environments.

Referring again to Figure 1, a second laser structure at the

rear of the transmit laser serves as a laser power monitor, providing a feedback signal for the laser drive current control circuit. A third laser structure is included on the second, other­wise unused, arm of the 3 dB coupler, to absorb unwanted light, and hence to further reduce internal crosstalk effects. This component may also be used as a laser to assist in the active alignment of the T P O N receive fiber during module assembly. Addit ional key subcomponents are the process tolerant cou­plers where we make use of a novel coupler design that is both polarization insensitive and tolerant to small variations in pro­cess parameters, material composition, and thickness.5 Access to the network is achieved at a single output port. Once di ­rected through the chip, telephony and broadband receive services are coupled into multi-mode and mono-mode fiber interfaces, respectively, and then into off-chip signal detectors.

C H O I C E OF INTEGRATION T E C H N O L O G Y

Integration of the key laser and waveguide subcomponents may be achieved by a number of different routes, inc luding large optical cavity (or similar twin guide), 6 butt join selec­tive growth, 7 and butt join non-selective growth (or growth over contoured substrates).8 For our work, the butt join se­lective growth approach was chosen as a l lowing a more versatile route for the integration of different design lasers, waveguides, signal detectors, and other subcomponents and hence for fabricating a w ide range of PICs using the same process technology. A n important feature is the use of semi-insulat ing (SI) substrates, as this enables the different active components w i th in the PIC to be electrically isolated f rom one another, a l lowing direct connection of the PICs to stan­dard laser dr ive and receive circuit blocks. The laser struc­tures fabricated on the SI process have the further advantage of h igh speed operation 4 due to min ima l bond pad capaci­tance. Ana log bandwidths in excess of 10 G H z are achieved, enabling the use of the PICs in h igh speed digital and RF analog applications.

Fabrication of the PICs is achieved by the use of a three-stage growth process, inc luding laser pr imary wafer growth, waveguide in f i l l , and laser overgrowth. The latter two growths are ach ieved by selective area, l o w pressure M O V P E , 7 wh ich is part icularly important for achieving re­producible, planar laser to waveguide butt joins. We are now incorporating M O V P E grown M Q W laser structures for improved power and enhanced high temperature opera­tion. This is important given that the PICs described here w i l l be expected to operate w i t h o u t the benef i t of thermocoolers in modules aimed at low cost, customer ac­cess applications. PIC processing employs we l l established integrated circuit photolithographic processes, including two level metall ization and poly imide isolated laser n- and p-contact tracks. Extensive use of methane/hydrogen dry etch­ing is made for fabricating the laser and waveguide stripes, wh ich leads to excellent etch uniformity across whole two-inch wafers. Methane/hydrogen etching is also used for producing electrical and optical isolation slots elsewhere on the wafer. Overa l l dimensions of the T P O N / B P O N PIC are: length=3.75 m m , width=0.75 m m , and thickness=0.12 m m , which al lows several hundred devices to be prepared per two-inch (5 cm) wafer. A photograph showing a close-up v iew of the laser to waveguide interface and laser contact arrangement for a typical PIC is presented in Figure 2.

OPTICS & PHOTONICS N E W S / M A R C H 1993 17

Photonic Integrated Circuits

P E R F O R M A N C E

A s an example of PIC performance, a set of characteristics taken for 1300/1530 n m bidirectional, broadband transceiver PICs 1 is shown in Table 1. These devices are similar to the T P O N / B P O N circuits b u t do not have the 3 dB coupler for direction mult iplexing.

A s a comparison, init ial characteristics for more recently prepared T P O N / B P O N PICs show network facet output powers in excess of 8 m W and lasing threshold currents as low as 18 m A . A l ight /current plot is shown in Figure 3. The other main characteristics of the T P O N / B P O N PICs are s imi­lar to those shown in Table 1, but w i th the addit ional 3 dB loss in the 1300 n m telephony signal path.

P A C K A G I N G

Al though not covered here i n any detail , the f inal key re­quirement for the use of PICs in real systems applications is the parallel development of suitable packaging where the challenge is to design a manufacturable, low cost, and highly stable environment in wh ich the PIC w i l l operate. For mod­ules fabricated at GEC-Marcon i , use is made of Si 'V' groove technology for network input /output and receive channel interfaces. For the T P O N / B P O N module, a telephony re­ceiver tile is inc luded wi th in the module whereas broad­band traffic is directed to an external receiver v ia a monomode

PARAMETER TEST

CONDITION

TYPICAL BEST UNITS

T H R E S H O L D P U L S E D 28 23 mA

C U R R E N T

P O W E R INTO 55 M A 270 570 μW

L E N S E D F I B E R C W DRIVE

D U P L E X E R ,

T H R O U G H

STATE

C R O S S T A L K

@ 1300 N M 14 19 dB

C H I P L O S S @ 1300 N M 4.7 3.4 d B

T H R O U G H - S T A T E @ 1530 NM 3.8 3 d B

C R O S S - S T A T E

WAVELENGTH 50MA

C W DRIVE

1289 -1 2 9 9 NM

S I D E M O D E " 2 8 3 5 d B

S U P P R E S S I O N R A T I O

Table 1. An example of PIC performance.

fiber interface at the 1530 n m output port on the PIC. Such a module containing first generation n-substrate T P O N / B P O N PICs has been successfully demonstrated in bidirectional telephony trials as part of the G P T L I M I T E D contribution to Brit ish Telecom's fiber to the home systems f ield trial estab­l ished in Bishops Stortford, U.K., dur ing 1991.

F U R T H E R D E V E L O P M E N T S

Current developments include the fabrication of similar de­sign PICs to that described above but w i th the benefit of on-chip signal detectors. A s described, the T P O N / B P O N PIC requires three fiber interfaces, two of wh ich employ single-mode fiber, the other mult i -mode. By incorporating signal detectors for the telephony and broadband receive channels, the number of critical fiber interfaces w i l l be reduced to one,

further reducing packaging cost. Incorporation of detectors w i l l also enable on wafer testing to be carried out. A t this stage, it is not thought l ikely that the PIC wou ld be further developed to include significant electronic functionality since we believe that a hybr id approach, using si l icon or possibly G a A s electronics and techniques such as solder bump inter­connection, w i l l lead to lower cost and higher performance modules than fu l l integration of electronics and optics onto the InP chip. This approach leads to the photonic/optoelec­tronic integration complementing the established circuit tech­nologies, rather than attempting to displace them.

We have described here, w i th specific examples of T P O N / B P O N and Broadband Bidirectional L ink PICs, a generic process suitable for the fabrication of a w ide range of diverse photonic circuits. Photonic integration technology is now sufficiently mature that PICs may be designed and prepared as practical solutions to real systems requirements, rather than just acting as technology demonstrators. A d ­vances currently i n development w i l l further impact the functionality, cost and reliabil i ty advantages offered by a PIC process. This w i l l expand the range of applications and increase volume requirements, both of wh ich are necessary for a new integrated circuit process to meet its fu l l potential of cost reduction.

A C K N O W L E D G M E N T S

The authors w ish to acknowledge the many colleagues at G M M T Caswel l invo lved i n the above work, wh ich was supported by G P T L I M I T E D and in part by R A C E B L N T , project number R1012; R A C E O M A N , project number R2073; and L I N K , M O C V D for integration.

P E T E R J. W I L L I A M S is Chief Physicist, Optoelectronics; P A U L

M . C H A R L E S is Department Manager, Optoelectronic Device Department; and A N D R E W C . C A R T E R is Senior Scientist, Op­toelectronics, at GEC-Marconi, Materials Technology Ltd., Towcester, Northants, England.

REFERENCES

1. P . M . Char les et al., " O E I C s for W D M transceiver modu les fabr icated by reactive ion e tch ing on semi - insu la t ing substrates," Electr. Lett. 28, 1992, 1084-1085.

2. C . M . Ragda le et al., "Integrated laser and a d d d r o p opt ica l mul t ip lexer for na r rowband wave leng th d i v i s i on mu l t i p lex ing , " Electr. Lett. 28, 1992, 712-713.

3. D .W. Fau lkner et al., "Op t i ca l ne tworks for local loop appl icat ions," J. L igh t . Tech. 7:11, 1989, 1741-1751.

4. P . M . Char les et al., "Integrateable h igh speed D F B lasers fabr icated o n semi - insu la t ing substrates," Electr. Lett. 27, 1991, 700-701.

5. R . G . Wa l ke r a n d A . C . Car ter , "Po la r i za t ion and process tolerant wave ­length dup lexers for integrated 1300/1530 n m b id i rec t iona l transceiver modu les , " E C O C / I O O C '91, poster paper We.PS2.2.

6. T .L . K o c h et al., "S imp le in- l ine b i -d i rec t iona l 1.5 μ m / 1 . 3 μ m transceiv­ers," 12th Int. Semiconductor Laser Conference Digest , September 1990. K-4 , 166.

7. J . T h o m p s o n et al., "Select ive area g rowth for dev ice integrat ion," J . C rys ta l G r o w t h 124, 1992, 227-234.

8. D . Remiens et al., " G a I n A s P / I n P integrated r idge laser w i t h a butt jointed transparent opt ica l wavegu ide fabr icated b y single-step meta lorganic vapor -phase epi taxy," J . A p p l . Phys . 68, 1990, 2450.

18 OPTICS & PHOTONICS N E W S / M A R C H 1993