operation.
6.2.2 Final configuration
O ur fin a l experim ental configuration (Fig. 6.5) consisted o f a mode-locked e x te rn a l g ra tin g la se r used in co n ju n ctio n w ith th e tra v e llin g -w a v e sem iconductor optical a m p lifie r. Several configurations o f th is type have been reported in the lite ra tu re . They were used fo r the in v e s tig a tio n o f gain com pression o f sem iconductor o ptical a m p lifie rs fo r both DC ^ and pulsed e le ctrica l drives^. They were also im plem ented to study c h irp in g and com pression o f optical pulses^. F in a lly , the use o f a sem iconductor o p tica l a m p lifie r to stabilise and shape the pulses produced by a mode-
locked sem iconductor laser has been investigated and reported in Ref. 6.
6.2.1 D escription o f the System
The exte rn a l ca vity laser consists o f the BTR L laser (ser. no. 27989) AR-
coated to 0.1%, M elles G rio t 2mm coupling sphere AR-coated to 0.25% and
a M ilton-R oy g ra tin g u tilis in g 12001/mm and blazed fo r 1.4pm. The optical b a n d w id th o f th is e xte rn a l g ra tin g c o n fig u ra tio n is estim ate d to be lOOGHz. The laser heath sin k is m ounted on a M a rlo w P e ltie r cooler, model M I1023T-02AC. The m icrowave connection to the laser is provided by a connectorised sem i-rigid coaxial cable bonded to the laser subm ount using a silv e r loaded epoxy. A ll m echanical components are assembled on a Photon C ontrol bench ra il providing variable ca vity le n g th from 3.75 to
15cm (corresponding to 4GHz and IG H z re p e titio n rates respectively).
The o u tp u t o f the laser is taken by an AR-coated lensed fib re , obtained fro m BTR L, and spliced to a B T& D optical iso la to r model O IC l 100-1500. The fib re coupler is a Sifam 22S15A50, p roviding equal s p littin g between tw o branches. One o u tp u t arm o f the coupler is spliced to a D iam ond connector, p ro vid in g the d ire ct o u tp u t fro m the e xte rn a l c a v ity laser, w h ile the other arm is spliced to a BT&D SO A 3100-1550 tra ve llin g-w a ve sem iconductor optical a m plifie r. The a m p lifie r is specified to provide 12dB gain fo r TE and 7dB fo r TM mode when biased a t 100mA. In o u r set-up i t is driven by the IL X LDX-3412 laser supply.
F or mode-locked operation, the laser is d rive n using the S ystran D onner 1618 synthesiser in conjunction w ith a M in i-C irc u its ZH L-4240 a m p lifie r and an M I F7117-05 microwave isolator. The m icrowave d rive is combined w ith the DC bias from IL X LDX-3207B using an HP 33150 bias-T.
A m p lifie d pulses from the mode-locked laser were m easured u sin g the
In ra d 5 -1 4 -L D a u to c o rre la to r. The H P L ig h tw a v e A n a ly s e r
(HP70000+70810A) was employed to display the RF spectra o f mode-locked o p tica l pulses before and a fte r a m p lific a tio n . GTE photodiode w ith a b andw idth o f 40GHz and the T ektronix CSA 803 w ith SD-32 sam pling head (50GHz bandw idth) were used to display the pulses.
6.2.2 C haracterization o f the System
The external cavity system was tested fo r two ca vity lengths whose round-
tr ip frequencies were in the v ic in ity o f 2GHz and 4GHz. The e xte rn a l
c a v ity were set-up using the lig h tw a ve analyser to m o n ito r the noise spectra o f the laser. For optim al alignm ent, the noise components a t the ro u n d -trip frequency and its harm onics are com pletely suppressed fo r a broad range o f the bias cu rre n t. We d id n o t have th e equipm ent to m easure the la sin g frequency o f the exte rn a l ca vity laser. The s o lita ry sem iconductor laser was presum ed to lase a t ~ 1.57pm. The e xte rn a l ca vity laser was expected to have a tu n in g range o f the order o f 40nm ^.
In our experim ents there was a need to detune the la sin g frequency o f the e x te rn a l c a v ity c o n fig u ra tio n in o rde r to o b ta in m a x im a l pulse a m p lifica tio n . From the BTR L data sheets, the gain peak o f the a m p lifie r
was positioned around 1.535pm a t the bias o f 100m A. T h is im p lie d th a t
the laser should be detuned to the shorter w avelength side o f its gain spectrum . D etuning o f the lasing frequency tow ards sh o rte r w avelengths can re s u lt in an increase in the d iffe re n tia l gain and a decrease in the lin e w id th enhancem ent factor o f the sem iconductor laser. There is also some e xp e rim e n ta l evidence w hich was reported in th e lite ra tu re to
in d ica te th a t th is type o f detuning can lead to pulse shortening in mode- locking®.
A set o f lig h t / cu rre n t curves presenting the operation o f the s o lita ry and
e xte rn a l ca vity laser is presented in Fig. 6.6. The o p tica l pow er o f the
external ca vity laser is plotted fo r both the DC and combined DC and RF d rive (m ode-locking) o f the laser. The la sin g frequency o f the e xte rn a l c a v ity la se r was detuned fo r m axim al pulse a m p lific a tio n . The o p tica l feedback was estim ated to be in the region between 5 and 7%.
6.2.2.1 4 GHz E xternal C avity
The external ca vity laser was set-up w ith a cavity le n g th corresponding to a 3.85 GHz ro u n d -trip frequency. The sem iconductor la se r was d rive n w ith a DC c u rre n t o f 30mA and RF signal o f a p p ro xim a te ly 0.8W . The sem iconductor o p tic a l a m p lifie r was biased a t 100m A. The best auto co rre la tio n trace o f the am plifie d optical signal is presented in Fig. 6.7. The fu ll w id th h a lf m axim um o f the trace is ll.S p s im p ly in g the p u lse w id th o f 8.2ps fo r a Gaussian pulse shape or 7.4ps fo r sech^ pulse. The absence o f the a u tocorrelation spike gives an in d ic a tio n o f proper m ode-locking. A comparison can be made w ith Fig. 6.2 w hich represents the case o f p a rtia l m ode-locking. No autocorrelation m easurem ents have been obtained from the d irect o u tp u t o f the mode-locked laser due to the lim ite d s e n s itiv ity o f the autocorrelator.
T y p ic a lly , a u to c o rre la tio n traces w ere 12 to 12.5ps lo n g . T h e y corresponded to the averaged oscilloscope traces o f 16ps u sing the GTE photodiode, SD-32 sam pling head and the sa m pling head extender. L ig h tw a ve analyser traces o f mode-locked pulses before and a fte r the
a m p lifie r are shown in Fig. 6.8a & 6.8b respectively. The average optical
power measured from the output fibre was 16 and 400|iW respectively. The fir s t value im p lie s an average o u tp u t power o f the m ode-locked la se r o f around 100|iW i f the coupling, in se rtio n loss and s p littin g loss are taken in to account.
R F + exter lal cavit r
6 0 - 5 0 - o 40
solii ary lasei
10 20 30 40 60
0 50 70 80
cu rre n t m A
Fig. 6.6
L ig h t / current characteristics o f the s o lita ry laser, DC driven and mode-locked external ca vity laser obtained from the output fibre. Extem sJ ca vity co nfiguration adjusted fo r m axim al a m p lific a tio n .I l l
m m
Ri
F ig . 6.7 Autocorrelation trace - am plified output mode-locking frequency 3.85 GHz
autocoirelator settings: scanning range 102ps
p a 1 5 E ' 8 : 5 7 1 1 . 0 3 . 1 9 9 2 RL - 1 0 . 0 0 dBm niCR S l a F R Q 1 9 . 2 B GH; RTTEH 1 dB 3 . 0 0 d B / D I V - 1 . 5 0 dB LI GHTWAVE O PT I CA L AUG PWR - 1 B . 0 dBm MARKER A .5B GHz .5 0 dB A 1
y
r
--- jA. ____u J START 2 . 7 0 GHz RB 3 . 0 0 MHz VB 3 0 0 kHz STOP 2 6 . 5 0 GHz ST 1 . 5 8 7 s e c a)m
1 9 : 2 7 : 5 0 1 1 . 0 3 . 1 9 9 2 A1 Mf IB TEN 1 d 00 d B / Q I G PWR - H RKER A I' : Li g h t w a v e' 1 . 7 7 dB OP T I CA L . 0 dBm ; . 2 B GHz . 7 7 dB ' ■ 1 : :ft
f
w ' W Y START 2 . 7 0 GHz STOP 2 6 . 5 0 GHz b) RB 3 . 0 0 MHz VB 3 0 0 kHz ST 1 . 5 8 7 s e cF ig . 6.8 HP lightw ave analyser traces: mode-locking a t 3.85GHz
a) direct output of the mode-locked laser b) am plified output
The effect o f d e tu n in g the m o d u la tin g frequency on th e p u ls e w id th , m easured fro m the a m p lifie d o u tp u t is presented in F ig . 6.9a. The shortest pulses were produced fo r sm all positive d e tuning, up to 5M Hz. The la se r was more sensitive to negative detuning, and the p u lse w id th w ould double fo r the reduction o f m odulating frequency o f 20M H z w ith respect to the ro u n d -trip frequency.
6.2.2.2 2 GHz E xternal C avity
The m ode-locking experim ent was repeated using a longer ca vity w ith a ro u n d -trip frequency o f 2.23GHz. The laser was biased a t 20m A and d rive n w ith an RF signal o f 0.8W. The optical a m p lifie r was biased a t 1 0 0m A, as in our previous experim ent. S am pling oscilloscope traces shorter th a n 16ps were measured fo r the am plified ou tp u t. A ty p ica l trace is presented in F ig. 6.10. The diagram shows a fu ll w id th a t a h a lf m axim um o f 15.4ps. The equivalent deconvolved p u lse w id th is 8.2ps. The average o p tica l power m easured from the d ire c t and a m p lifie d ou tp u ts were 12.5 and 250pW respectively. In th is experim ent th e re was some u n c e rta in ty in the determ ination o f the exact ro u n d -trip frequency o f the exte rn a l ca vity laser. However, the frequency detu n in g curve, presented in Fig. 6.9b, has the same tre n d as the curve obtained fo r the shorter ca vity (Fig. 6.9a).
M ode-locking was also achieved when the laser was d rive n a t the second harm onic o f the ro u n d -trip frequency (4.46GHz). In th is case the DC bias o f the laser was increased to 30mA. The measured pulses w ere n e a rly id e n tic a l to the pulses produced by m ode-locking o f the sh o rte r exte rn a l c a v ity s tru c tu re . H ow ever, i t seemed th a t the o p e ra tio n was m ore sensitive to the cavity m isalignm ent because o f the appearance o f 2.23GHz components in the RF spectrum . This effect was m ore pronounced fo r both positive and negative detuning, and sometimes the RF spectra w hich resem bled fundam ental m ode-locking a t 2.23GHz were observed. Pulse in s ta b ilitie s in harm onically driven external ca vity sem iconductor lasers
-60 -40 -20 0 20 40 60 80 frequency detuning M H z 40 -60 -40 -20 0 20 40 60 a) b) frequency detuning M H z
F ig . 6.9 D etuning o f the mode-locking frequency: v a ria tio n o f the pulsew idth measured from the a m plifie d o u tp u t
a) mode-locking a t 3.85GHz b) m ode-locking a t 2.23GHz
in D 2 3 0 k 4 A M P L I F I E D M L D L ® 2 G H z R E P 1 3 / 3 / 9 2 Qh FWHM 1 5 . 4 ps
me
0F ig . 6.10 Sam pling oscilloscope trace: am plified ou tp u t o f the mode-locked laser detected by the 40 GHz GTE photodiode
as w e ll as period doubling (only fo r positive frequency detuning) have been observed and reported in the literature^»
C h a p te r 7