Dr. Lee Wing Kee
Chinese University of Hong Kong

Applications of lasers in the following areas will be covered:

Fiber optics – by exploiting total internal reflection in a glass fiber, one can transmit optical signal to a place that is very far away.

(Fig. 1)

Advantages:
(1) Low loss: , is called attenuation coefficient, x is the distance traveled, and is the intensity at x = 0. Signal amplification is required in every 50 km, while electrical signal in every several km.

(3) Potentially high data rate: nowadays, ultrashort pulse lasers can generate laser pulses as short as sec, or 10 femtoseconds (fs), a potential fast data rate of ~ bits/sec. (Encyclopedia Britannica contains ~ bits of information).

Technical challenge: high quality glass and fiber (concentrations of impurities should be extremely low, high homogeneity so that absorption and scattering losses are minimized.)

Laser source: semiconductor laser (very compact), long life, easily modulated( In order to transmit signal, signal has to be on and off represented by "0"s and "1". That is why it is necessary to modulate the laser light.).

Choice of wavelength:
1.3 and 1.5, because lowest loss can be achieved as shown in Fig. 25.

Explain dB:

dB is a measure of attenuation. Let I and be the intensities at x and at x = 0 (signal has propagated a distance x), respectively, then I is said be decreased by dB of .

(Fig 2)

• Can carry a lot of information e.g. the communication optical cable (from H.K. to Japan) has only 7 optical fibers, but can carry thousands of phone calls simultaneously.
• Signal becomes weak after ~50 km. (i.e. low loss). [Question: How about electrical signals in a telephone line?]

Remark:
In a certain type of optical fiber, the refractive index is not uniform as shown in Fig. 26. This type of fiber can also guide a light ray (see exercise)

(Fig 3)

• Cut metals in a clean way and fast.
• Laser welding.
Use high power laser or Nd:YAG laser. (CW, 5000 —›10,000 W), it is a lot of power!!
• Drilling tiny holes in materials becomes an important application.

2.2.1 Welding

• Advantages:
(1) hard-to-get-to places can be welded, and
(2) extremely localized heating compared to other welding methods. (A laser beam can be focused to a spot size of ~100- diameter or less.)

• Requirement: High power lasers such as (wavelength 10.6 ) or Nd:YAG (wavelength 1.064 or 0.532 ) lasers. YAG stands for Yttrium Aluminium Garnet.

(For a large frame CW laser, output power can reach 5000W !)

Both and Nd:YAG laser can be operated in CW or pulsed mode. In pulsed mode, a laser typically gives pulses of 1 ns in width while Nd:YAG lasers can give pulses of ~10 ns or 0.1 ns in width depending on the method by which the pulses are generated.

Remark: laser radiation can generate heat efficiently in solids.

2.2.2 Laser Etching

Etching small pattern on materials is very common in modern technology. For example integrated circuit (IC) industry requires etching of small patterns (dimension ~0.15 micron).

Choice of Laser: Short wavelength (UV) radiations from Excimer Lasers.

Basic principle: A short duration (~ sec) UV photons have enough energy to break the chemical bonds rapidly and creates a mini-explosion that ejects materials.

(Fig 4 Etching using an excimer laser beam.)

Important Characteristics:
(a) Edges are very sharp.
(b) No burn.
(c) Extremely small patterns can be etched.

Example: Used in integrated circuits manufacturing and micro-machines. (Micro-machine is a machine, such as a motor, whose size is only hundreds of microns. For example, such a machine can be put into your blood vessels to carry out a task, such as cleaning.)

• Leveling
• Digging tunnels (exploit: light travels in straight line).

There are many applications including cancer treatment, removal of pigments, etc. Here we just give only two examples below. Readers can always go to the Web for more applications.

4.1 Corneal sculpting

Remark: It can be considered as one type of etching done on human tissue.

Remarkable fact: ArF laser beam can cut a single living cell leaving an otherwise undamaged half behind.

Penetration depth in the cornea: 4 for 193-nm radiation 48 for 248-nm radiation.

Etching of corneal tissue using 194-nm ArF laser radiation occurs by photo-decomposition of the peptide bonds. Typical tissue removal rate saturates at ~ 0.5 per pulse. By counting the pulses the depth control for excimer laser cutting of the cornea is extremely precise.

4.2 Excimer laser angioplastry

Build up of calcified plaque within the arteries leads to constriction that may eventually lead to a heart attack.

Clearing of such a blockage can be achieved by burning through using an excimer laser as shown in Fig. 5.

(Fig 5)

5.1 Laser spectroscopy

In spectroscopy, there are typically two cases: (1) a sample emits light waves consist of many different wavelengths, and (2) a sample is excited by a light beam (usually a laser beam), the sample then remit light waves consisting of, in many cases, many different wavelengths. These wavelengths are then separated and recorded by a device called spectrometer. The result is called a spectrum. Certain properties of the sample can then be deduced by the spectrum.

In case (2), one often needs to change the wavelength of the laser beam used for exciting the sample. Here we introduce two methods for accomplish this goal.

(A) Frequency conversion

(1) Higher harmonics generation -- higher harmonic generation is an important method for generating different frequencies. In electromagnetism, it is well known that

,

where P is the polarization (that is electric dipole moment per unit volume) and E is the electric field and the susceptibility. This equation is good for weak electric fields. For an intense laser pulse, E is so large that higher order terms become important, so that

,

Since we are only interested in time dependence, we only consider

,

By using simple trigonometry identities, we see P will acquire terms like

, and , etc.

We know that as P varies sinusoidally, electromagnetic (EM) wave is emitted. The frequency of the EM wave is same as that of the oscillation of P. EM waves of 2, 3, etc., are be generated. Efficient generation of higher harmonics normally exploit some “nonlinear” crystals meaning that values , , etc. of these crystals are higher than those of common crystals.


Here is a question to ponder: can one use a piece of glass to generate higher harmonics?

Remark: crystals used for higher harmonic conversion have to be transparent at the wavelength of that harmonic, and, of course, the fundamental too).

(2) Parametric conversion

Consider an intense light beam of frequency , and a light beam of frequency , (> ). These two light waves interact inside a crystal. Result is that light beam of frequency get more intense (i.e. amplified), and that of frequency becomes weaker, and another beam of frequency is also generated, and
= + .

Of course, momentum has to be conserved also.
This process is known as parametric conversion.


(B) Laser cooling of atoms

Basic principle: Atoms are produced from an ion beam or oven, then irradiated by using a laser beam propagating opposite to the momentum of the atoms. The frequency of the laser is tuned to one of the absorption lines of the atom. If an atom absorbs a laser photon and then the atom re-emits a photon. As a result, its momentum is generally reduced because the direction of the re-emitted photon is random. Even though, initially, the momentum of a photon is small compared to that of the atom, repeating this process will slow down the atom enough and allow one to trap many atoms in an “optical trap” and then cool the trapped atoms down to K!

left: Slow down atoms using one laser beam. right: Trapping atoms using six laser beams.

(Fig 6 Experimental setup for laser cooling and trapping of atoms.)

Important consequences
Cooling atoms down to such a low temperature has important consequences in spectroscopy, for example, one does not have to worry about Doppler shift any more. To understand the role of Doppler shift in spectroscopy, let as look at the diagram below.

Ideally, when the atoms are not moving, the emission line is very sharp as indicated by (a). If the atoms (molecules, such as those in a gas laser) are moving, each atom would have a different velocity, the emission is boarden because of Doppler effect. The frequency measurement becomes less accurate.

(Fig 7)

Recently, scientists managed to cool down tens of thousands of Rb and Li atoms and experimentally found an exotic phenomenon known as Bose-Einstein condensation in this system. Bose-Einstein condensation was predicted ~70 years ago by Bose and had eluded experimentalists for many years. More recently, Prof. Steven Chu of Stanford University, Prof. Claude Cohen-Tannoudji and another scientist shared the Nobel Prize for their contributions to cooling of atoms and new record of accuracy in time keeping.

Note: Accuracy of ~1 part in , or 1 second error in ~ years. Accuracy of a conventional atomic clock is one part in .

(C) Laser levitation and manipulation of tiny objects

A photon carries momentum. If light wave is absorbed or reflected by a surface, there is momentum change in the photon momentum and thus a force exerting on the surface. This force is known as radiation pressure, which was predicted by Maxwell more than a century ago. However, the magnitude of radiation is extremely small even for lasers. So experiments for measuring radiation pressure had been extremely difficult before the advent of lasers. In 1972, Professor A. Ashkin, then at Bell Labs, demonstrated that a focused laser beam can be exploited to levitate a micro-meter sized transparent dielectric sphere.

(Fig. 8 Experimental set-up for studying elastic light scattering from a levitated droplet.)

Note that the key word is transparent, if the sphere is metallic, that is shinny, it cannot be levitated. (WHY?) If the sphere is a droplet whose liquid absorbs the laser light strongly, then it cannot be levitated (WHY?).

Exercise: Estimate the force required to levitate a water micro-droplet of 20 microns in diameter. (Answer: order of nano-gram.)

5.2 Lasers in life science research

(A) Cell puncture -- a new technique in biological research
A tiny hole ~ micron in diameter can be “drilled’ by a laser pulse in a cell membrane. This technique allows the interior of the cell be studied or chemicals injected directly into the cell. It is an important tool for drug research, and many others. A pulsed Nd:YAG laser is being used for this purpose.

(B) Human DNA sequencing -- project of the Century

Several years ago, it was calculated that sequencing all the human genome would take 1000 years. Almost ten years ago, only 170,000 base pairs were identified. The total number of base pairs is ~. Based on capillary electrophoresis and laser induced fluorescence, a method has been developed to expedite the job. Speed ~30 bases/min.

Remark: the human genome map is expected to complete in year 2003.

Remark: A hologram can be cut into small pieces and each piece can be used to generate the virtual or real images, but with a degradation of the sharpness of the images (WHY?).

Experimental setup:

( Fig. 9 (a) A laser beam is split into two, one irradiates the object and the other irradiates the photographic plate after reflecting from a mirror. (b) Reconstruction of 3D image. )

A set of corner cubes has to be placed on the surface of the moon and it (a set of 100 pieces) was taken there in July 1969 by the Apollo 11 astronauts. [A set of 100 pieces, and a set of 300 pieces were placed on the moon by the Apollo 14 (Feb 1971) and 15 (July 1971) astronauts, respectively.]

A corner cube is a corner cut from a glass cube along the dashed lines in Fig. 10(a) below. That is why it is called a corner cube.

It has a very interesting characteristic: an incident light ray will be reflected three times by the corner cube and the out coming ray is always parallel to the incident ray, independent of the orientation of the corner cube. Question: can we use a mirror rather than a corner cube? Why?

(Fig 10)

We have mentioned ranging already, here we focus on light detection.

(A) Basic ideas

Send a laser pulse into the sky and monitor the return pulse due to scattering.

(1) If there is a layer of aerosols in the atmosphere, the returning pulse is stronger compared to that from a clean atmosphere. The density of the layer of aerosol as a function of height can be deduced by analyzing the shape of the returning pulse.

(2) If there is a layer of pollutants, e.g. , in the atmosphere and the laser frequency is tuned to one that can be strongly absorbed by a molecule, then the returning pulse will be much weaker compared to that from a clean atmosphere. The amount of as a function of height can also be deduced. Of course, by choosing suitable laser wavelengths, one can remotely detect many types of pollutants.

(Fig 11 Schematic diagram of a LIDAR system.)

9.1 Adaptive optics

Optical system in which controllable elements, such as deformable mirrors, are used to correct for optical wave-front distortions such as those caused by turbulence in the viewing path. The distorted light beam to be corrected is reflected by the deformable mirrors (Fig. 12 shows how a mirror can be deformed slightly) and sampled by a beam splitter. Wave front sensors then look for wave-front distortions and then inform the control unit to deform the mirrors so that wave-front distortions are corrected. Result: a blurred image of a star, due to turbulence in the atmosphere, becomes sharp!

(Fig 12 A deformable mirror.)

9.2 Guide Star (Source: Physics Today, February 1992)

Fig. 13 shows a scheme exploiting an artificial guide star. The laser beam is tuned to the 589-nm sodium line to the mesospheric atomic sodium layer. Due to resonant scattering(When the incident light frequency is equal to the energy difference between two energy levels of an atom, scattering of light turns out to be particularly strong, which is called resonant scattering.), the scattered light is quite intense and hence appears as a star and therefore called a guide star. This “star” passes through the atmosphere and surely distorted by the turbulence. But we know the original shape of the laser spot and therefore can correct for the distortion and hence the distortions of the real stars whose light rays come to the telescope through the same path of the guide star.

Remark: One can also exploit Rayleigh scattering as an artificial beacon. But its performance is much inferior to that of a sodium guide star.

(Fig 13)