Lasers 

Laser -- Light Amplification by Stimulated Emission of
Radiation

The recipe for a laser & Energy levels for Gain Medium

Recipe:
1.  Two mirrors are required.  One with 100% reflectivity and the other with less than 100%
reflectivity.
2.  A gain medium -- a material which has an energy level structure with an upper lasing level that is
long lived to make an inversion (more electrons in the upper lasing level than the lower lasing level).  This is the material whose electrons are excited in order to make the amplification of the
light.  These may be gases (argon, HeNe), solids (YAG), semiconductors, or liquids (dye lasers).
3.  Pump -- a mechanism for pumping electrons in the gain medium up into the upper lasing level.
This may be done with a flashlamp, electrical current, or another laser.
4.  Spontaneous emission -- spontaneous emission occurs when an electron in an excited state
jumps back down to a lower energy level giving off a photon.  The photon can go in  any direction
and has no particular phase.
5. Spontaneous emission occurs in direction of the mirrors and is caught between the mirrors --
bouncing back and forth from mirror to mirror, traveling through the gain medium.
6.  Stimulated emission -- stimulated emission occurs when a photon "tickles" or interacts with an
electron in the upper lasing energy level, stimulating it to emit a photon and jump to the lower lasing
level.  The stimulated photon is emitted in the EXACT same direction as the initial photon and with
the EXACT same phase.  THEREFORE, the total wave (electric field) that is traveling in the laser
cavity is now increased in amplitude (constructive interference).
7.  We have Amplified the light going through the gain medium when bunches of electrons make this
stimulated transition down to the lower lasing energy level.

Energy Levels for Gain Medium

The best energy level system is the FOUR level system.

The highest energy level has a short lifetime (electrons will leave quickly from this state) and the
electrons are excited there by the pump.
The upper lasing energy level is the next lowest in energy.  Electrons quickly drop to the upper
lasing level from the highest level.
The upper lasing level has a long lifetime (electrons will stay there awhile).
The lower lasing level has a short lifetime (electrons drop to lowest energy level from here).  We
don't want many electrons in the lower lasing level or else they could absorb light from the cavity and
decrease the total amplitude of the laser light.

Basic components of digital optical communication system:

• Original message
• Digitize it into a series of binary bits (0s and 1s)
• Transmit the bits
• Detect at other end
• Decode and recreate message

• Laser -- creates light -- 830, 1300, and 1550 nm are most used wavelengths
• Modulator -- adds the information to the light (turns it on and off to represent 0s and 1s)
• Fibers -- transmit light -- losses are measured in decibels (a 3 dB loss is 50% amplitude loss)
• Amplifiers -- build up the signal if there are too many losses or the message must travel long distances
• Photodetector -- converts optical signal into an electrical pulse (light ON = pulse out)
• Demodulator and Computer -- re-creates signal from pulses

• Multiplexers -- mix signals of different wavelengths together (each wavelength carries a different message) and then sends it through fiber
• Switches -- re-route various signals to the correct receiver
• De-multiplexers -- separate signals of different wavelengths apart

DIGITIZATION:  Taking an analog signal (a phone message for instance) and converting it to a digital signal in which binary code is used.   This means that each amplitude of a signal is converted to a 16 or 24 bit number (16 or 24 0s and 1s representing the amplitude).

Digitization of Sound Waves:

       A sound wave is sampled at 44 kHz and at each time interval the height of the wave is measured.  It is then given a value that reflects its
relative amplitude to a zero value. This amplitude is turned into a binary number.

       Binary numbers are determined by how many powers of 2 you need to make up the particular amplitude that portion of the sound wave had. So 4 bit binary number only goes up to a maximum amplitude value of 15 which comes from 1 x 2^3 + 1 x 2^2 + 1 x 2^1+ 1 x 2^0.  Here 4 powers of 2 were used producing a digital signal of 1111.  In CDs, for example, every amplitude is represented by a 16 or 24 bit number which uses 16 (or 24) powers of 2 to find the amplitude. This means that the maximum amplitude can be given a value of over 65000 (3 million).

      Once a digital number is assigned to the amplitude, a computer then programs the modulator to turn the laser beam off and on mimicking the 0s and 1s of the binary number.

An Aside:  Digitization -- A Real World Example -- your CD player:

  Once a digital number is assigned to the amplitude, a computer designs the CD master disc for
pressing the ones we buy.  A series of bumps
are put on the master disc which will translate to pits on the final CD.  The CD is made by
pressing the master disc onto the plastic one, coating it with aluminum and then adding a final coat
of plastic.

The CD:

      The CD has pits and lands about 0.5 microns wide on tracks and the tracks are spaced about
1.6 microns apart.  The height is around 150 nm or so.

        A diode laser beam is focused onto the CD from below. It makes a spot  around 0.8 microns
in diameter (thus the inter track spacing).  When the beam is reflected off the CD, it is sent to a
detector.  Here the size of the signal (due to the amplitude of the light) is used to determine if
there is a pit, a land, or a transition between a pit and a land occuring. Transitions are marked by a
big change in reflected signal size and thus are given the designation of a 1while lands and pits
keep the same reflectance and are given the designation of 0.

 Playback:

        The computer chip reads the 16 1s and 0s off from the detector and translates them into an
amplitude (this occurs in the Digital to Analog
Converter [DAC]). The amplitudes are put together to form a wave which is smoothed via a low
pass filter and sent off to the amplifier for the
speakers again as an analog signal.

Special LInks to get you started:

Total Internal Reflection:  See Group 17's website

Physics Today Article:

http://www.aip.org/pt/vol-53/iss-9/p30.html

Fiber Optic Communications;

http://www.science.org.au/nova/021/021key.htm
http://www.commspecial.com/fiberguide-print.htm
http://www.netoptics.com/5.html

Solitons and Amplifiers in Optical Communications:

Back to digital optical communications:
Optical Components needed to accomplish  digitization of the signal:

• Laser -- creates light -- 830, 1300, and 1550 nm are most used wavelengths
• Modulator -- adds the information to the light (turns it on and off to represent 0s and 1s)

    Figure 2 shows the parts of a laser light source with a modulator attached.

1.  Laser -- diode laser

Quantum wells -- gain medium
Index of wells is greater than surrounding medium
Inversion is created via electrical current
One mirror is normal
Second mirror is a Distributed Bragg Reflector (DBR) -- special mirror made of layers of differing index of refraction --each layer reflects a portion of the light and it constructively interferes to create a back-reflected beam (for light of the correct wavelength).

2.  Tuning the Laser -- need to pick out different wavelengths in order to send more than one message in the fiber

Use DBR mirror.
By sending a current into the DBR mirror, the indices of refraction change in the one type of layer.  This is effectively the same as changing the length of the different layers and thus a different wavelength will be the correct length to undergo constructive interference of the back reflected wave. We now have a new wavelength in the laser which will be stimulated and experience gain.

3.  Laser Amplifier -- Like the diode laser, it is made of quantum wells.

Light from the laser makes a single pass through this gain medium.  The excited electrons are stimulated to emit radiation that is in phase and in the same direction as the laser beam and it amplifies the beam - makes it more intense.  However, unlike a laser, the beam only travels through the amplifier ONE TIME.  The amplification is about 100.

4.  Modulator --- Here is where the digitized signal is added to the beam.

The modulator is an electro-absorber.  This means that if we put a 2 V bias across the material it becomes opaque to the laser beam and none of the light gets through (it is all absorbed).  IF we do not put a voltage across the modulator, the beam gets through.  THUS, the 0s and 1s are written on the laser beam just like on the CD.  The light is turned off and on and the pulses carry this information (like pits and lands on the CD) to the receiver on the other end.

1.  What is the 6 bit binary number for an amplitude of:
a)  28
b)  15
c)  30

2.  A CD/DVD player would be able to store more information if the laser used in the player had a _______ wavelength.  This is because these wavelengths ________.

3.  For total internal reflection to occur, a light beam incident on an interface, must obey the following expression.  This mathematical equation tells us the first angle at which total internal reflection will occur.
n₁ sin q₁ = n₂ sin 90
a) If n₁ = 1.4 and n₂ = 1.1, what is q₁?

b)  Which of the following incident angles will be totally internally reflected in the system outlined above?
35^o
40 ^o
45 ^o
50 ^o
55 ^o
60 ^o
65 ^o
70 ^o
4.  Match the optical components which are needed for each step in a digital optical communication system.
Basics:                                                                      Optical Components:
Digitize Message                                                                 Amplifier
Imprint Message                                                                  Photodiode and Computer
Send Digitized Message                                                      Computer and Modulator
Detect Message                                                                  Laser
Decode Message                                                                  Fiber and Lenses

5.  Explain how more than one message can be sent down an optical fiber.  What optical components are necessary specifically to accomplish this?

6.  Which statement about diode lasers is correct?
a)  Diode lasers do not have a gain medium.
b)  The quantum wells in a diode laser have a lower index of refraction than the surrounding media and thus they guide the light out the side of the laser.
c)  The distributed Bragg reflector (DBR mirror) has many layers, each of which reflects a small portion of the light, and the sum due to constructive interference creates a strongly reflecting beam.
d)  The DBR mirror changes its reflectivity when the temperature changes.
e)  The electrons injected into the diode laser via the pumping current undergo spontaneous emission to amplify the light traversing the cavity.

7.  The electro-absorbing modulator creates the optically digitized signal  Explain how this happens.

8.  Optical fibers lose light by a number of mechanisms.  List these methods of light loss and describe each.

9.  Describe the difference/similarities between optical amplifiers right by the diode laser and farther up the optical fiber.

10.  Alexander Graham Bell first thought of using a modulated light beam for a telephone.  How does his old idea compare with modern digital optical communications systems?

11.  How does a CD player read 0s and 1s?

12.  What is the difference between "normal" analog and digital TV?

14. If a signal traveling down a fiber loses 3 dB per 4 km, how much signal will remain after 16 km?  Recall that 3dB loss is 50 %.

15. If you want to amplify the signal from Q14, how much amplification would be necessary?

16. Modern amplifiers are able to amplify the incoming signal by about 100 times, will one amplifier be enough for the example we are considering?

17. How far apart would such an amplifier need to be spaced to maintain the signal's intensity?