The Photonics Quiz

A Gaussian laser beam is

(a) a beam with Gaussian intensity profile

(b) a beam with Gaussian intensity profile and plane wavefronts in the focus

(c) a beam with Gaussian intensity profile and plane wavefronts at any position

How may absorption/emission cycles does a Nd³⁺ ion on the beam axis of a Nd:YAG laser do per second?

(a) roughly 20 000

(b) roughly 20 millions

(c) That is strongly dependent on factors like the output power, pump intensity, etc.

Quasi-three-level lasers are less power-efficient

(a) in any case, as the reabsorption on the laser transition wastes energy

(b) for low pump intensities

(c) for unidirectional pumping

(d) for too high doping concentration and/or a too long laser crystal

Why does the use of a laser crystal with a very broad wavelength tuning range typically lead to a high laser threshold?

(a) because a broad emission bandwith leads to strong ASE

(b) because a broad emission bandwith leads to a smaller σ–τ product

(c) because the broad bandwidth results from microscopic disorder, which increases scattering losses

Does a saturable absorber as used for passive Q switching of a laser necessarily lead to a substantially reduced power efficiency?

(a) No, it doesn't. If its saturation energy is very low, not much energy is required to saturate the absorber.

(b) No, it doesn't, if one photon can saturate several absorber ions.

(c) Yes, the principle of using saturable absorption unavoidably degrades the efficiency, since the absorption must be similarly strong as the initial laser gain.

Which factors lead to short pulses from a passively Q-switched laser?

(a) a short laser resonator

(b) a short upper-state lifetime

(c) a small amount of saturable absorption

(d) a large amount of saturable absorption

(e) a high pump intensity

(f) a laser crystal with high σ–τ product

When a pulse is generated in a Q-switched solid-state laser, at which level does gain saturation become strong?

(a) when the intensity reaches the saturation intensity

(b) when the temporally integrated intracavity power reaches the saturation energy

(c) when the temporally integrated intracavity power reaches the stored energy

Which factors help to minimize the pulse duration from a Q-switched laser?

(a) a short upper-state lifetime

(b) a high product of upper-state lifetime and emission cross section

(c) a high pump intensity and duration

(d) a short laser resonator

Why are semiconductor lasers not suitable for pulse generation via Q switching?

(a) because semiconductors have very short carrier lifetimes

(b) because semiconductor gain media have very small gain saturation energies

(c) because one cannot integrate a Q-switch into a semiconductor chip

Is self-starting passive mode locking easier to achieve for short laser resonators?

(a) yes, because this increases absorber saturation in the early phase of pulse generation

(b) no, because this reduces the ratio of peak power to average power and thus leads to weaker absorber saturation

Are the lines in the spectrum of a continuous-wave laser exactly equidistant?

(a) yes, the line spacing is the inverse round-trip time, which is constant

(b) no, because there is generally some amount of chromatic dispersion

(c) no, because higher-order transverse modes have modified resonance frequencies (assuming that such modes are excited)

Why are the lines in the spectrum of a mode-locked laser exactly equidistant?

(a) because of dispersion compensation in the laser resonator

(b) because the pulse repetition rate is constant

(c) because the saturable absorber or active modelocker enforces this condition

How does absorption of the idler wave in the crystal of an OPO or OPA affect the efficiency of the device?

(a) not at all, if only the signal output is of interest; only heating of the crystal could hurt the efficiency by disturbing the phase matching

(b) the efficiency can be severely degraded, even without thermal effects

Two single-mode fibers with different core sizes but same numerical aperture are spliced together. How about the splice loss?

(a) It is lower for light coming from the fiber with smaller core, compared to the other direction.

(b) It does not depend on the direction.

Two multimode fibers with different core sizes but same numerical aperture are spliced together. Light is sent into the fiber with the smaller core. How about the splice loss?

(a) The losses at the splice can be substantial, since the mode sets do not fit.

(b) There cannot be significant splice losses, since all light gets into the core of the second fiber.

A figure-of-eight laser is

(a) a laser from a company with that name

(b) a mode-locked fiber laser containing two coupled ring resonators

(c) a mode-locked fiber laser containing a nonlinear loop mirror

(d) a ring laser which is folded such that the beam crosses itself in the middle

(e) a laser with a specially shaped beam profile

Large mode area single-mode fibers usually have a rather small numerical aperture,

(a) because this is required for achieving single-mode guidance

(b) because this helps to limit bend losses

(c) because this makes it possible to attenuate higher-order modes via bending

(d) because that makes them compatible with standard single-mode fibers

Superfluorescence and superluminescence are

(a) two words for exactly the same phenomenon

(b) two different phenomena

A DFB laser is

(a) a miniature semiconductor laser

(b) a narrow-linewidth laser

(c) a dual-frequency Brillouin fiber laser

A fiber lens is

(a) a lens with the shape of an optical fiber

(b) an optimized lens for coupling light into single-mode fibers

(c) a lens made from amorphous carbon fiber material

Parametric fluorescence is

(a) a quantum-mechanical effect occurring in OPOs and OPAs

(b) responsible for the finite threshold pump power of an OPO

(c) an effect in an OPA which is analogous to ASE in a laser amplifier

In a Nd:YAG laser, amplified spontaneous emission (ASE) is usually much weaker than in a fiber laser,

(a) since there is no waveguide to confine the randomly emitted fluorescence

(b) because Nd:YAG has a lower emission cross-section

(c) because the emission bandwidth of Nd:YAG is smaller

(d) because Nd:YAG is a four-level laser medium

Does a poor beam quality have a detrimental impact on the conversion efficiency of a frequency doubler?

(a) yes, mostly in cases with critical phase matching, much less with noncritical phase matching

(b) in any case, independent of phase-matching details

(c) not at all, as long as tight focusing is still possible

The use of quasi-phase matching (QPM) in a nonlinear frequency conversion device

(a) leads to a reduced effective nonlinear coefficient

(b) is less practical in cases with high phase mismatch

(c) can be applied with nearly all nonlinear crystal materials

(d) allows for an increased phase-matching bandwidth

Which of the following statements concerning frequency doubling of ultrashort pulses are correct?

(a) Shorter pump pulses can lead to a higher conversion efficiency.

(b) Shorter pump pulses can lead to a lower conversion efficiency.

(c) Crystal damage is more likely to be a problem for shorter pulses.

(d) For efficient conversion, one needs to check both group velocity mismatch and the phase-matching bandwidth.

(e) The phase-matching bandwidth is irrelevant as long as the pulses are not chirped, so that their instantaneous frequency is constant.

Is the following statement true: What happens in a degenerate optical parametric oscillator (OPO) is exactly the time-reversed version of resonant frequency doubling.

(a) Yes, why not?

(b) No, these processes rely on different nonlinearities.

(c) No, these processes differ in terms of phase matching.

(d) No, as a time-reversed frequency doubler would have two input waves, whereas the OPO has only one.

The zero-point fluctuations (vacuum fluctuations) of the electromagnetic field

(a) are associated with a certain small density of photons (one photon per mode), which cannot be further reduced because quantum mechanics predicts some balance of absorption and emission processes for any absorber material

(b) are occurring when no photons at all are present

Erbium-doped fiber amplifiers are

(a) the most efficient fiber amplifiers, disregarding only some devices based on prohibitively expensive types of fibers

(b) the type of fiber amplifier offering the lowest noise figure

(c) operating in the wavelength region where standard single-mode fibers have lowest losses

(d) suitable for transparent optical networks, since they simply transmit any input signals when not being pumped

Assume that a periodic picosecond pulse train is generated with a gain-switched laser diode. Does the spectrum of the pulse train have the shape of a frequency comb, consisting of sharp lines?

(a) Yes, it does, because periodic pulse trains always have such a spectrum.

(b) No, it doesn't, because the pulses are not mutually coherent.

(c) This depends on the quality of the electronic driver of the laser diode.

Assume that we have two identical ultrashort optical pulses, one following the other with a delay of several times the pulse duration. What will be the optical spectrum of that double pulse?

(a) The spectrum is not the same as for the single pulses; it has additional interference fringes.

(b) The spectrum will be the same as for each single pulse, except if the pulses are so close that their wings interfere with each other.

If we attenuate a laser beam (e.g. by sending it through an absorber), will that modify its relative intensity noise?

(a) No. The power fluctuations relative to the average power will remain unchanged.

(b) Yes. The relative power fluctuations will increase, since we have to consider the squares of the power deviations.

(c) It depends. Relative intensity noise near the shot noise level will be increased, much stronger intensity noise will not.