Chapter 2
Experimental Techniques
For the experiments we use laser cooled Cs-atoms as working atom. The ob-servation region around the nanofiber is very small comparable to the wavelength, so atoms can quickly pass through the observation region. Assuming an observation region crossing the nanofiber to be 1 µm, for room temperature (300 K) atoms, the transit time will be 5 ns, that is smaller than the spontaneous emission time 30 ns. It will result in transit time broadening. But if we can cool down atoms down to 100 µK, we can realize much longer transit time around 10 µs. So we combine both thin fiber technology and laser cooling technology for experimental realization.
2.1 Tapered Optical Fiber Technology for Realizing
vari-ous technical limitations like, precise control of gas flow is required to maintain a suitable temperature, disturbance due to the air current can seriously affect the repro-ducibility and the purity of the gas is most crucial to realize quality submicron tapers.
CO2 laser technique is much cleaner than the flame method but it has a limitation on the minimum achievable diameter since the amount of energy absorbed reduces as the fiber becomes thinner. A smarter way to overcome this limitation is to use a mi-crofurnace with CO2 laser. In this technique the CO2 laser heats a sapphire crystal rather than the fiber itself and this heated sapphire crystal acts as the microfurnace [27].
We use a similar microfurnace technique but in our case a resistively heated ce-ramic heater acts as the microfurnace. We use commercial fiber coupler production system (NTT-AT, FCI-7011) for making tapered fibers. These machines are usually designed for making tapered fibers with diameter of several micrometers. But for realizing submicron tapered fibers we modify the pulling-speed of this machine to match our purpose. The temperature and pulling speed are controlled precisely using computer program and during preparation the transmission is monitored simultane-ously at a wavelength of 850 nm.
Figure 2.1(a) shows the photograph of the tapered fiber preparation setup. The whole setup is installed inside a clean booth which is equipped with a dust filter which continuously flows dust free air from the top. The preparation setup consists of production unit and control unit. The control unit consists of heater-power supply,
a)
b)
CeramicHeater
Photo Diode Stripper
Stripper
Splicer Fiber
Pulling Stage
End Cutter
Tip Cleaner
P C
Control Unit
Clean Booth
Production Unit
Figure 2.1: Tapered fiber preparation. (a) Photograph of tapered fiber preparation setup installed inside a clean booth equipped with dust filter. It consists of production unit and control unit. The production unit consists of ceramic heater, pulling stage and photo diode for transmission measurement. The control unit consists of heater power supply, digital multimeter, controller and everything is computer controlled.
(b) Photograph showing the production unit in more details and showing various tools used for tapered fiber preparation.
digital multimeter, controller and computer. A detailed photograph of the production unit is shown in Fig. 2.1(b) showing various tools used for tapered fiber preparation.
The production unit contains the ceramic heater, pulling stage and photo diode for transmission measurement. Some of the important tools for tapered fiber production includes mechanical strippers, fiber end cutter, fiber end cleaner and fiber splicing machine.
We use single mode optical fibers (SMF) (Fujikura, SM.10/125.04.UV) for producing tapered optical fibers. The diameter of a conventional single mode bare fiber is 250µm with polymer coating. To avoid contamination we strip the polymer coating before preparation. We use chemical stripping technique for clean stripping without making any scratch on the fiber. Without the polymer the diameter is 125 µm which include both clad and core. The core diameter is around 9µm for a cut-off
wavelength of 1.3µm. For adiabatic tapering we use a two step pulling process. For each step the temperature and the pulling speed are well chosen to realize the required diameter and minimum loss. After the first step pulling the minimum diameter of the taper is several micrometer. Then after the second step the minimum diameter is few hundred nanometer, smaller than the wavelength. The nanofiber is located at the waist of the tapered optical fiber. In this region the core is almost vanishing and these are basically vacuum clad fibers and the clad of the original fiber serves as the core.
Adiabatic tapering is crucial for maintaining single mode propagation through the tapered region and to realize minimum loss. The general single mode condition
[37] for an optical fiber is given by 2πaλ p
n2core −n2clad <2.405, whereais the core radius,λis the wavelength of the propagating light andncore andncladare the refrac-tive indices of the core and clad of the fiber respecrefrac-tively. The thinnest region of the tapered fiber can support only the fundamental mode. But in the tapered region some higher order modes can be excited and it will result in loss in the thinnest region.
However for adiabatic tapering the light can propagate mainly in the fundamental mode. The adiabatic tapering condition is given by drdz6 2πr [β1(z)−β2(z)], where
dr
dz denotes the local taper angle, r is the local core radius and β1(z) and β2(z)are the local propagation constants for the fundamental mode and the next higher-order mode respectively [65, 66, 67, 68].
Apart from adiabatic tapering the most crucial point for realizing low-loss sub-micron tapers is to avoid contamination by dust. For subwavelength tapers a major part of the guided mode lies outside the fiber, in the evanescent region. So any dust particle can completely destroy the transmission. And most of the light can escape due to scattering from the dust particle. So the most important technical requirement is to prepare the nanofibers in dust free environment and to protect the nanofiber from dust contamination until installing into the vacuum chamber.
The basic characteristics of the tapered fibers are shown in Fig. 2.2. The trans-mission characteristics of tapered fiber with different waist diameter are shown in Fig. 2.2(a). The transmission is measured for 850 nm light. However the original single mode fiber has a cutoff of 1.3µm. So to measure the transmission of the
fun--6 -4 -2 0 2 4 6 0
200 400 600 800 1000
Diameter [nm]
Length [mm]
800 600 400 200
0 20 40 60 80 100
Tr ansmission [%]
Minimum Diameter [nm]
a)
b)
SEM Image
c)
Figure 2.2: Characteristics of tapered optical fiber. (a) Transmission of 850 nm light through tapered fibers with different minimum diameters. (b) Diameter variation along the length of tapered fibers with minimum diameter 400 nm. The different curves correspond to different nanofiber samples but all of them are prepared under same production conditions. (c) SEM image of 400 nm diameter tapered fiber.
damental mode we splice a single mode fiber with cutoff of 850 nm before launching light into the fiber which acts as a single-mode filter. Fiber diameter down to 100 nm is possible. But the loss increases rapidly at around 200 nm diameter. But for fibers with diameter around 400 nm, we could realize quite good transmission of around 90% and for the experiments we use fibers of diameter 400 nm as it satisfies the op-timum condition for coupling of Cs-atom fluorescence into the guided mode [30].
After preparation the nanofiber samples are collected in a metal substrate, for diam-eter measurement in a scanning electron microscope (SEM). The diamdiam-eter variation along the length for a 400 nm fiber is shown in Fig. 2.2(b) and an SEM image of such a fiber is shown in Fig. 2.2(c). The different curves in Fig. 2.2(b) correspond to different nanofiber samples but all of them are prepared under same production con-ditions. The minimum diameter is reproduced with an error of±10%. The minimum diameter is almost uniform for 2 mm at the waist having a variation within 5%. Such uniformity is realistic for the experiments since the cloud of cold atoms have simi-lar dimension. The tapering region is 4 cm on either side. That satisfies the adiabatic tapering condition. Regarding the surface roughness, we have not seen any irregular-ities within a resolution of 30 nm. Using such a technique nanofibers with diameter of 400 nm are routinely reproduced with a transmission of 90%.
In recent years tapered optical fiber tips are also produced using chemical etch-ing method and are widely used for scannetch-ing near-field optical microscopy (SNOM) [69]. However, it should be noted that such chemical technique is useful only for
multimode applications and are not suitable for producing low-loss adiabatically ta-pered single mode fiber.