OPTICAL SPECTRUMBYMICHAEL ANGELO M. DIMAON

OPTICAL SPECTRUM•  the decomposition of the power or energy of light according to different wavelengths or optical frequencies•"optical radiation" refers to electromagnetic radiation in the wavelength range between 100 nm and 1 mm.•a spectrum of electromagnetic radiation encompassing the  infrared, visible, and ultraviolet regions.

WAVELENGTH RANGES OF ELECTROMAGNETIC RADIATION

OPTICAL SPECTRUM• Optical spectra are a result of quantum transitions between the ener

gy levels of atoms, molecules, solids, and liquids.• Optical spectra are registered by photographic and photoelectric me

ans. Other methods and instruments may also be used, for example, quantum counters in the ultraviolet region and thermocouples and bolometers in the infrared region. Spectra may be observed visually in the visible region.

CLASSIFICATIONS OF OPTICAL SPECTRA

ACCORDING TO APPEARANCE:• 1. Line Spectra -consist of separate spectral lines corresponding to discrete values of λ( a line always has a finite width corresponding to a narrow range of λ.)• 2. Band Spectra -made up of separate bands, each covering a certain range of λ• 3. Continuous Spectra• -Continuous spectra cover a large range of λ

4 TYPES OF OPTICAL SPECTRA1.Emission Spectra2.Absorption Spectra3.Scattered-light Spectra4.Reflection Spectra

1. EMISSION SPECTRA• Emission spectra correspond to allowed transitions fr

om upper energy levels to lower levels• To obtain emission spectra, the radiation from a light

 source is separated into  its various wavelength components by means of a spectroscopic instrument. An emission spectrum can be characterized by the function f(λ) giving the energy distribution of the emitted light with respect to wavelength ʎ.

2. ABSORPTION SPECTRA

• obtained by passing light through a substance and then separating the light into its wavelength components. • for absorption spectra, k(λ), the

fraction of light energy absorbed at each wavelength

3. SCATTERED-LIGHT SPECTRA

• obtained by passing light through a substance and then separating the light into its wavelength components. • for scattered-light spectra, α(λ), the fraction of

light energy scattered at each wavelength

4. REFLECTION SPECTRA

• obtained by passing light through a substance and then separating the light into its wavelength components. • for reflection spectra, R(ʎ), the fraction of light

energy reflected at each wavelength. 

• In the case of the scattering of monochromatic light of wavelength ʎ0, the Raman spectrum obtained is characterized by the energy distribution of the scattered light with respect to the changed wavelengths ʎ ≠ ʎ0 [f’(ʎ)]. Thus, any spectrum can be characterized by some function f(λ) giving the absolute or relative energy distribution with respect to ʎ. Here, the energy is considered over a certain range of ʎ. The function f(λ) can be replaced by the function Φ(v) giving the energy distribution with  respect to frequency v = c/ʎ, where c is the speed of light. The energy is then considered over a certain range of v.

•The appearance of an optical spectrum depends on the state of the substance. If for a given temperature the substance is in a state of thermodynamic equilibrium with the radiation, the substance emits a continuous spectrum whose energy distribution with respect to λ or v is given by Planck’s radiation law. Usually, however, the substance is not in thermodynamic equilibrium with the radiation, and the optical spectrum can have various forms. Atoms, for example, are characterized by line spectra produced by quantum transitions between electronic energy levels. Simple molecules have band spectra resulting from transitions between electronic, vibrational, and rotational energy levels 

• For optical spectra, to different regions of λ or, consequently, of v there correspond different photon energies hv = ℰ1-ℰ2, where h is Planck’s constant and ℰ1 and ℰ2 are the energy levels between which the transition occurs. Table 1 gives for the three regions of electromagnetic waves in optical spectra the approximate ranges of wavelengths ʎ, frequencies v, wave numbers vie, photonenergies hv, and temperatures T. Here, T is the temperature characterizing the photon energy in accordance with the equation kT = hv, where k is the Boltzmann constant.

Table 1. Ranges of various quantities characterizing the regions of opticalspectra

Spectral region

ʎ(μ m) v (sec–1) v/c (cm–1) hv (eV) T (°K)

infrared

103–0.74 3.0 × 1011–4.0 ×1014

10–1.35 × 104

1.25 × 10–3–1.7

14–2.0 × 104

Visible 0.74–0.40 4 × 1014–7.5 ×1014

1.35 × 104–2.5× 104

1.7–3.1 2.0 × 104–3.6× 104

Ultraviolet

0.40–0.001 7.5 × 1014–3.0 ×1016

2.5 × 104–106

3.1–125 3.6 × 104–1.4× 106

VISIBLE SPECTRUM

Color Wavelength Frequency Photon energyviolet 380–450 nm 668–789 THz 2.75–3.26 eVblue 450–495 nm 606–668 THz 2.50–2.75 eV

green 495–570 nm 526–606 THz 2.17–2.50 eVyellow 570–590 nm 508–526 THz 2.10–2.17 eVorange 590–620 nm 484–508 THz 2.00–2.10 eV

red 620–750 nm 400–484 THz 1.65–2.00 eV

Color Wavelength Frequency Photon energy

violet 380–450 nm 668–789 THz 2.75–3.26 eV

blue 450–495 nm 606–668 THz 2.50–2.75 eV

green 495–570 nm 526–606 THz 2.17–2.50 eV

yellow 570–590 nm 508–526 THz 2.10–2.17 eV

orange 590–620 nm 484–508 THz 2.00–2.10 eV

red 620–750 nm 400–484 THz 1.65–2.00 eV

OPTICAL SPECTRUM ANALYSIS

• Optical spectrum analysis is the measurement of optical power as a function of wavelength. Applications include testing laser and LED light sources for spectral purity and power distribution, as well as testing transmission characteristics of optical devices.

• The spectral width of a light source is an important parameter in fiber-optic communication systems due to chromatic dispersion, which occurs in the fiber and limits the modulation bandwidth of the system. The effect of chromatic dispersion can be seen in the time domain as pulse broadening of a digital waveform. Since chromatic dispersion is a function of the spectral width of the light source, narrow spectral widths are desirable for high-speed communication systems.