be exactly e(|ual to the photo energy ratio, i.e., E(A2)/E(A i) = w j/w ,. If so, how woiihl an inappropriate energy ratio affect the conversion efficiencyY f ig. 7.4 shows nuinerlcally sirriiilated results for the case of E(A2) - A xE (A i) W2/W1, where A = i, 1.5, and 2. We can see th at as the factor A increases, the ])eak of the conversion efficiency curve moves towards the larger beam waist.
7.4 E xperim ent
The main problem in the LBO NCPM SFG experim ents was the low ])ulse energy of the pum p lasers. For example, the maximum output power of the EG Q-swit.ched laser at this tim e was ~4.5 m.I. If one divides the pum p laser into two parts in the ratio shown in Fig. 7.1 and uses one part to pum]> the K TP OPO , the expected signal output of the K TP OPO will be ~0.73 m.I (assum ing r/opo=35% ). From Fig. 7.2 (a), it can be seen th at the optim um focusing condition for this case (E( A, )< 1 m.) ) is W', ~0.05 mm. For such a tightly focused condition, three ex]>erimental configurations were investigated in our work. The first one was a directly focused configuration, wheie the LBO was placed in the outp u t path of the O PO, and the pum p and signal beams were then focused using the same focusing lens. The second one was a folded configuration, where the pum p laser f)eam was divided into two parts by a 90" beam splitter, one part going to pumj) the OPO, while the other part propagated directly into the LBO crystal. The third one was an intracavity configuration, where the LBO crystal was placed inside of the OPO cavity.
7 .4 .1 T h e first c o n fig u r a tio n
We know that an advantage of the scheme K TP OPO(ty])e III) -f LB O SFG (type I) is that the signal and puiii]) waves can be directly focused into the tyj)e I LBO SFG because they have the same polarisation direction. Therefore, the first SFG configuration is the m ost sim])le strncture of the three. The experim ental set-u]) is shown in Fig. 7.5.
OPO
LBO SFG
pum p laser
CH APTER 7. TU NABLE RED LIGHT GENERATION 163
In this configuration, two lenses, Fi and F^, were used to focus the pum p and signal beams for the SFG process. The function of Fi was to collimate the two beam s, hence F i was placed at a distance d% ~ /i from the centre of the OPO cavity. F2 then acted as a focusing lens, and was placed as close as possible to F%. The LBO crystal was then placed at the image point (focusing point) of F2.
Here, both the AO and EO Q-switched pum p lasers were used for the experim ent.
(1 ) E O Q -sw itc h e d p u m p la se r: In this case, the two focusing lenses, F i and F2, were chosen to be /i= 5 0 0 m m, f2 = 125 mm. Using the optical ray tracing m ethod, the pum p
and signal beam waists at the focal point were found to be mm, =0.056
mm respectively. W hen the pum p laser outp ut was 4 m J, 12 ns, the signal output from the NCPM K TP OPO(single-pass-purnp) wa,s 0.54 inJ, and 0.2 m J red light at 0.623 fun was detected. The experim entally achieved conversion efficiency was, obviously, much lower than tire theoretically predicted upper-lim it value (see the curve with the param eter of E (A i)=0.5 m.I given in Fig. 7.3).
I
(2 ) A O Q -sw itc h e d p um p la se r: In this case, the focal lengths of the two focusing lenses were chosen to be /i= /2 = 1 2 5 mm. From calculation, the focused pum p and signal
beam waists can be found to be: 14^^=0.0716 mm, I4^^=0.1 mm respectively. W hen the | laser was pum ped on each facet with 5 w atts of diode light and Q-swltched at a 3 kHz '] repetition rate, the average o u tp ut power of the signal light from the intracavity K T P j OPO was 100 rnW with 15~20 ns pulse width. The depleted pum p o u tpu t was m easured
to be 200 m W, but its peak power was found to be only half th a t of the signal peak power, and its pulse profile was wide (~ 70 ns) and flat. 14 m W average o u tp ut power of the red light was detected in this case.
Obviously, one problem in this configura,tion was th at the pum p and signal pulse profiles are uncontrollable. M oreover, for a highly efficient K TP O PO , there is inevitably a poor tem poral overlap f)etween the signal and pum p pulses due to the pum p depletion. In fact, for the case of using the EO Q-switched pum p laser, a large portion of the pum p energy was wasted because of the low outpu t power of the signal light; and for the case of using the AO Q-switched pum p laser, the signal energy was wasted because the highly depleted pum p pulse.
C H A P T E R 7. T U N A B L E RED L IG H T G E N E R A T IO N 7 .4 .2 T h e s e c o n d c o n f ig u ra tio n Fl(pump) Ft 1()4 LBO S i c; dl(pump) d2(pump) pum p laser OPO
Figure 7.(J: Experim ental set-up of the second construction for red light generation. The exi)erimental set-up of the second SFG configuration is shown in Fig. 7.fi, wheie only the EO Q-switched laser was used for the experim ent. In this scheme, the pump beam was separated into two parts for the ])urpose of using one ])art to pump the K TP OPO while keeping the other part undepleted for the SFG. In front of the LBO crystal, the pump and signal beams were collimated individually by two lenses, which were F, and Fi(pum ])). The collimated beams were then combined by a 45" dichroic mirroi and focused down by the lens F2. Ty])ical param eters of this focusing system are indicated in Fig. 7.2, the focused pump and signal waists, according to calculation, were: f/;',^,=0.04S mm, <n',^=ü.ü58 mm respectively. In this experim ent, by using a 45" beam -splitter we firstly divided 45 mW average j)ump j)ower into two parts in the ratio of 2:1, i.e., 50 mW went to pump t he K TP OPO. and 15 mW went to the LBO crystal. In a double-pass-pum p OPO scheme, 9 mW of signal o utput power was achieved at the 50 mW pump level. The energy ratio between the signal and the ])um]> lor the LBO SFG was Pp/P,ç = 1 5/9~ i.f), and this is close to the photon energy ratio, A*./Ap = 1.47. Behind a ]>rism, a maximum of 0 mW of red light was detected. Taking the losses on each optical surface^ into account, it can be shown th at the conversion efficiency of the SFG process had reached to 50%, and the overall conversion efficiency from the |>mn]) to the red was more than 15%. Foi such a low pump energy level, the theoretical curves given in Fig. 7.5 indicate th at the potential m aximnm conversion efficiency of the SFG process alone is ~40% .
There are several advantages in this SFG configuration. First, it is convenient to be able ‘ All the optical components used here, including the locusing lenses, crystal and the prism, were not coated
CHAPTER 7. TUNABLE RED LIGHT GENERATION 105
1,0 adjust energy partitioning between the pump and signal. Secondly, although there is always 1~2 ns delay of signal ])ulse due to the signal field build up, with e((ual distances between the beam splitter and the centre of the LBO for both legs, the two beams can be well overlapped tem porally with no pum p depletion problem. Thirdly, it allows the use of a double-pass-pum p configuration for the O PO, and hence increases the overall efficiency. As our theoretical predictions show, at high pum p energy levels with improved coatings, the m axim um conversion efficiency of the LBO SFG process can be as high as 55%, and the overall conversion efficiency can reach to 40% (if the conversion efficiency of the K TP OPO is 40%.). This has been proved by L. Marshall et al. [1] recently, who re])orted a similar SFG scheme for uv generation at 0.280 fim with 55% conversion efficiency for the SFG and 25% for the overall conversion efficiency when the pump laser output energy was 50 m.J.
7 .4 .3 T h e th ir d c o n fig u r a tio n
F