Chapter 2 Apparatus
2.1 Resonant light
2.1.1 The doubled 1560 nm system
The recent appearance of inexpensive high-power fiber amplifiers at the telecommu- nications wavelengths near 1560 nm was a serendipitous event from the point of view of the atomic physicist. These amplifiers (available up to > 10 W) combined with the availability of highly efficient frequency-doubling technology [110–112] allowed the construction of a novel source of narrowband 780 nm light, developed at JPL3 for use in the laser cooling of rubidium.
This apparatus exploits a clever arrangement devised in order to overcome some limits placed on doubling efficiency. The traditional expression for frequency doubling involves the intensity of a plane wave after propagating through a short crystal of length L:
I2ω(x, y, z =L)∝ω2L2Iω2(x, y,0) (2.1.1)
Typically, however, the benefits of focusing the laser through a short crystal far outweigh that of sending an unfocused beam through progressively longer and more unwieldy crystals. The cost of focusing the beam is a reduction of the doubling’s length dependence to linearity. The production of frequency-doubled light through a single doubling crystal is then specified in terms of the normalized conversion efficiency
¯
η, such that:
P2ω = ¯ηLPω2 (2.1.2)
The specific value forη will of course depend on the internal physics of the crystal as well as the intensity profile of the focused laser; as might be intuited, the ideal profile is nearly that determined by so-called confocal focusing, where the length of the crystal L is matched to the confocal parameter—twice the Rayleigh rangeπw20/λ. A clever way to recover the lostL2 dependence is to place several short crystals in series,
3Described in a 2003 paper [113], reprinted as Appendix D; doubled light had been used previously to lock a 1560 nm laser using rubidium lines, but this only generatedµW-level powers at 780 nm [114].
effectively duplicating the condition of frequency doubling in an optical waveguide, hence the literature sobriquet of lens waveguiding in a cascade of crystals[115]. The doubled (scalar) field in such a cascade of crystals is given by superposition:
E2ω =
N
X
i=1
E2ω,i (2.1.3)
Combined with Eq. 2.1.2, we obtain the doubled power output, given N identical crystals of length L:
P2ω =
N
X
i=1
pP2ω,i
!2
= ¯ηLN2Pω2 (2.1.4)
Thus, for a two-crystal cascade, one would expect a factor-of-four gain compared to operating with a single crystal, given ideal conditions and proper phase matching.
The seed for the Yb/Er-doped fiber amplifier (IPG Photonics) is an external- cavity diode laser (ECDL) in the Littman-Metcalf configuration (New Focus Vortex) providing 10 mW of narrowband light at 1560 nm. The Vortex is tunable over 50 GHz, has a linewidth of 300 kHz at 50 ms, and can be feedback-controlled using piezoelectric tuning of the external cavity as well as fast control of the diode laser current. The amplifier is internally pumped with 980 nm light and is turnkey-operable at output powers up to 5W of 1560 nm light of ostensibly similar spectral profile to that of the diode laser. This beam is confocally focused through a crystal of periodically poled lithium niobate (PPLN) as depicted in Fig. 2.1 (photographically in App. D).
After the first crystal, the beam is recollimated and it then traverses an adjustable length before being confocally focused a second time through another crystal. The adjustable length serves to ensure that the 2ω light from the first crystal arrives at the second crystal in phase with the fundamental—a full wave retardation in air between 780 nm and 1560 nm being approximately 50 cm. Phase matching within the 5 cm long crystal is permitted by the periodic poling and is achieved using precision temperature control of the crystals themselves—the poling period used was 19 µm,
33
1560 beam dump Dichroic splitter
To experiment
EDFA 5W PPLN crystal 2
PPLN crystal 1
ECDL
Figure 2.1: Diagram of the doubled-1560 nm fiber amplifier system.
which phase-matched at 100◦C.
The two-crystal cascade when optimally aligned at full power yielded just lower than 1 W of 780 nm power in our best measurement. In an earlier measurement of the low power limit, the quadratic dependence on input power was observed, resulting in efficiences of 4.6 mW/(W2·cm) in a denatured two-crystal configuration4, and 5.6 mW/(W2·cm) in the cascade. Ideally we would see a factor-of-two difference here, but several factors come into play that reduce the performance to a 20% bonus: most obviously, the insertion of loss of each crystal (4%) and the intra–cascade optical elements, but also spatial mismatch in the second crystal between the doubled beam and the fundamental due to imperfect focusing. Deviations from the simple quadratic formula of Eq. 2.1.2 are of course expected at higher power levels due to depletion of the source beam.
In terms of power generation capability, this setup is remarkable. It easily sur- passed the previous source of infrared power in our laboratory, the Ti:Sapph, at less than half the cost, and seems to be astonishingly scaleable in terms of fiber-amplifier power and (with care) number of crystals. The utility of having an unused excess of 3 W of 1560 nm light available for use is also tantalizing, either for separate use with
4By which we mean a modified two-crystal cascade where light at 2ω is removed between the crystals using a dichroic beamsplitter in order to isolate the ‘seeding’ effect.
other crystals or for use as a far-off resonant dipole trap.