Is It Possible to Construct a Continuous Wave Cw Laser

Real-Time Fluorescence Lifetime Imaging and FRET Using Fast-Gated Image Intensifiers

GLEN REDFORD , ROBERT M. CLEGG , in Molecular Imaging, 2005

3.4.3 Light modulators

Continuous-wave (CW) laser light can be modulated to give the high-frequency structured light needed for frequency-domain measurements. There are several different types of light modulators, the primary ones being acousto-optic modulators (AOMs) and Pockel's cell modulators. AOMs are inexpensive and easy to set up (Piston et al., 1989), but they usually only operate at particular frequencies and suffer from low depth of modulation and low throughput (amount of the source light in the modulated beam). Often great care is required to keep the AOM phase stable and the temperature must be carefully controlled. A.A. (St-Rémy-Lès-Chevreuse) makes AOMs for many different applications. New Pockel's cell systems are able to meet the high-frequency requirements and operate at different frequencies. They have excellent depth of modulation and all of the light is modulated (two beams are formed, with 50% of the input CW light in each beam, the second beam could be used as a reference for systems that do not need calibration). They suffer from phase drift over a longer time period, but this can be resolved in the data analysis. Conoptics, Inc., makes several different modulation systems that have the high-frequency response needed for frequency-domain measurements. Pockel's cell systems are more expensive then AOMs. However, it is possible to use inexpensive incoherent sources (lamps), which also offer wide wavelength ranges, with a Pockel's cell; the main difficulty is getting sufficient intensity through the small aperture of the Pockel's cell.

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Photoacoustic Probes for In Vivo Imaging

Vipul Gujrati , Vasilis Ntziachristos , in Methods in Enzymology, 2021

2.5 In vivo photothermal therapy

The goal of this experiment is to verify that near infrared (NIR) light irradiation can generate localized heat (in the tumor region) due to presence of OMV^Mel. Melanin generates strong heat to induce necrosis in the tumor region and retards tumor progression (Fig. 7). Greater heating is expected if OMV^Mel is injected directly into the tumor (i.t.) compared to the tail vein (i.v.).

Fig. 7. In vivo photothermal therapy (A) and tumor growth curve (B).

Reproduced with the permission from Gujrati, V., Prakash, J., Malekzadeh-Najafabadi, J., Stiel, A., Klemm, U., Mettenleiter, G., et al. (2019). Bioengineered bacterial vesicles as biological nano-heaters for optoacoustic imaging. Nature Communications, 10(1), 1–10. http://creativecommons.org/licenses/by/4.0/.

Equipment:

1.

Continuous wave laser (1.5  W   cm^{−   2}, 800   nm)

2.

IR thermal camera (e.g., FLIR i60)

3.

Isoflurane and oxygen supply (for anesthesia)

4.

Sterile insulin syringe

5.

Cell incubator (37   °C in 5% CO₂)

Cell line:

1.

4 T1 mammary gland carcinoma (CRL-2539, ATCC, Manassas, VA, USA)

Animals:

1.

Female athymic FoxN1 nude 6-week old mice

Note: Prepare the tumor animal model as discussed in the previous section of Section 2.4 imaging.

Procedure:

1.

Anesthetize the mouse in an isoflurane chamber, and maintain the anesthesia and oxygen flow through the nasal route

2.

Using an insulin syringe, inject the OMVs sample (~   75   μg) intravenously through the tail vein or intratumorally as per the study design

3.

At the predetermined time point (based on biodistribution MSOT information), expose the tumor region to a continuous wave laser (1.5   W   cm^{−   2}, 800   nm) for 6   min

4.

Record the tumor surface temperature during light irradiation using an IR thermal camera

5.

Record the tumor growth rate and animal body weight at fixed intervals (usually animals are sacrificed when tumor size reaches ~   1.5   cm).

6.

Compare the tumor growth retardation in OMV^Mel treated animals with all controls

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Clinical Applications of Fiberoptic Laser Systems

Abraham Katzir , in Lasers and Optical Fibers in Medicine, 1993

Laser Lithotripsy Using Laser Endoscopes

The first clinical trials involved CW laser beams which were sent through optical fibers. They vaporized the urinary calculi, rather than fragmenting them. This method is likely to cause severe thermal damage and is potentially unsafe. The first experiments with short-pulse Q-switched Nd: YAG lasers were done in the mid-1980s in Germany (Schmidt-Kloiber et al., 1985). They observed the plasma formation when nanosecond pulses of high peak power were focused on stones under water. Later (Hofmann et al., 1988) these researchers reported on the use of this technique for fragmenting urinary calculi.

The first experiments using flashlamp-pumped dye lasers were performed by Watson, Dretler and others (Dretler et al., 1987) Clinical trials were conducted using a 9–11 Fr rigid endoscope (urethroscope) and quartz fibers of diameter 0.2–0.3 mm. The success rate in these trials was high with practically no complications. Later, flexible ureteroscopes were also successfully used. This paved the way for wider clinical use of laser lithotripsy.

Methods identical to those described above may also apply to the fragmentation of kidney (i.e., renal) stones. In this case, the kidney pelvis may be punctured and a rigid endoscope (nephroscope) may be inserted and advanced to the vicinity of the stones. A power fiber is then inserted into the endoscope and used to transmit Nd: YAG or dye laser energy.

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Organizational Cell Biology

Y. Wu , ... E. Stefani , in Encyclopedia of Cell Biology, 2016

Continuous-Wave STED Microscopy

Continuous-wave STED microscopy uses a powerful continuous-wave (CW) laser source for depletion (Willig et al., 2007). The excitation source can be either CW or pulsed. It does not involve the complexity of temporal alignment in pulsed STED microscopy, and a CW laser source is more affordable than a pulsed laser source with similar optical power. The disadvantage of CW STED microscopy is that it requires higher average optical power to reach the same resolution as pulsed STED microscopy. This is obvious when the excitation light is pulsed and eqn [3] holds. The average irradiance of a CW depletion laser source needs to be I _dep constantly to reach the resolution d _STED; whereas for a pulsed depletion source the irradiance I _dep only lasts for its pulse duration T _w , and the average irradiance is I _dep T _w R, where R is the laser repetition rate. Therefore, the average depletion power needed in CW STED microscopy is (T _W R)⁻¹ times higher than in pulsed STED microscopy. Compared with using a common Ti:Sapphire laser with a repetition rate of 80   MHz laser and 400   ps pulse duration for depletion, CW STED microscopy needs 30 times higher depletion power to reach the same resolution. Such powerful CW laser sources are often not available, and therefore CW STED microscopes usually reach lower resolution than pulsed ones. Plus, high depletion power induces excessive photobleaching that can permanently destroy the fluorescent dyes. If the excitation is also CW, then excitation and depletion constantly compete against each other, which also results in higher depletion power requirement (Willig et al., 2007).

To summarize, compared with pulse STED microscopy, CW STED microscopy has the advantage of being technically simpler and working with less-expensive laser sources. However, its resolution is usually lower and it suffers from rapid photobleaching.

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Generation of single photons using semiconductor quantum dots

A.J. Shields , ... B.E. Kardynal , in Nano-Physics & Bio-Electronics: A New Odyssey, 2002

3.3 CW excitation

Photoluminescence was also collected after excitation of the mesa by a CW laser of similar energy. Such measurements revealed the same excitonic transitions, with identical photon energies (within experimental error) and similar linewidths as for pulsed laser excitation described above. Again the exciton line was found to have a linear dependence upon laser power, while for the biexciton it was again approximately quadratic, as shown in Fig. 5.

Fig. 5. Integrated intensities of PL lines as a function of power under CW laser excitation. Dotted (dashed) line shows the gradient associated with linear (quadratic) power dependence. The solid lines show calculated power dependence of X and X ₂ as a function of the rate of absorbtion of photons close to the dot.

The laser power dependence of the exciton intensities measured with pulsed and CW laser excitation differ at higher laser powers. This is because the exciton state of the dot can capture a second electron-hole pair for intense CW excitation before the emission of the exciton photon. For this reason the exciton emission weakens at the highest CW laser powers, while the maximum intensity of the biexciton is considerably stronger than that of the exciton for CW excitation.

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Surfaces, Laser Coating of

P.B. Comita , R.J. von Gutfeld , in Encyclopedia of Materials: Science and Technology, 2001

3.1 Laser Electroplating and Electrodeless Plating

Laser-enhanced plating investigations have used both pulsed and continuous wave (CW) lasers. The CW work has predominantly used the argon ion laser with power levels ranging from around 10   mW to around 20   W with maximum incident power density on the order of 10⁵ W   cm⁻². Pulsed lasers include frequency-doubled YAG and dye lasers with pulse widths in the nanosecond range. Laser-enhanced plating falls into two categories: electroplating utilizing an applied external EMF and electrodeless plating which occurs without an externally applied potential. Laser-enhanced electroplating generally requires a two- or three-electrode system (the third electrode serving as a reference electrode). Here, background plating occurs over the entire cathode due to the applied potential. However, since the irradiated region plates much more rapidly, the method can be used to delineate a pattern with back etching to remove the thin, unwanted background. Laser-patterned linewidths as small as 2   μm have been obtained. An example of the electrodeless type of deposition is laser-enhanced electroless plating which uses plating solutions formulated with their own reducing agent and requires a catalytic surface to produce plating. A local temperature increase causes a very substantial increase in the local deposition rate since the reaction is thermally activated. The plating rate R is given by an Arrhenius expression, R=A exp(−E/kT), with A a constant, E the energy of activation, k Boltzmann's constant and T the absolute temperature. Experiments with electroless nickel using focused argon ion laser light have resulted in plating rate up to 10⁴ times the rate observed for growth without laser irradiation (von Gutfeld et al. 1982). This large enhancement is due to the fact that the rate at ambient temperature is extremely low, about 0.01   nms⁻¹. Absolute laser-plating rates as high as 80   nms⁻¹ have been observed with electroless nickel.

A more intriguing example of electrodeless plating is thermally driven exchange plating, or laser plating via the "thermobattery effect" (Puippe et al. 1981). This type of plating was first demonstrated on a microscopic basis using the laser to irradiate a local area of a nickel thin film on glass, submerged in a copper sulfate solution. The laser-exposed surface became copper plated but to an extent far greater than that which is expected from simple immersion plating. Details of this mechanism were analyzed for copper ions plating onto a copper surface. It was found that the local temperature rise produced by the absorbed laser energy causes a positive shift in equilibrium potential of the heated Cu/Cu²⁺ interface, causing it to become cathodic. Some of the unheated (unirradiated) copper dissolves, in an amount equal to the number of ions plated, resulting in no net global change in the number of copper ions in solution. Thermal-exchange plating has recently been used to repair small defects on circuit boards, replacing the laser with local heat dissipation produced by an ac current passed through defective copper lines (Chen 1990). A third laser-enhanced electrodeless plating mechanism is attributed to disproportionation or dissociation of the electrolyte. Under high laser heat, certain complexed solutions, for example, gold–cyanide, have been observed to dissociate giving rise to depositions even on insulators, such as polyimide, quartz and glass.

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Multiphoton Excitation Microscopy and Spectroscopy of Cells, Tissues and Human Skin In Vivo

BARRY R. MASTERS , ... ENRICO GRATTON , in Fluorescent and Luminescent Probes for Biological Activity (Second Edition), 1999

31.4.4 Photodamage

For the single-photon process, fluorescence excitation typically uses continuous wave lasers and requires an average power of about 100 μW. For the two-photon microscope, lasers with 100 MHz and 100 fs pulses are often used and requires an average power of about 10 mW. This corresponds to a peak power on the order of 1 kW. The use of a laser pulse picker can significantly decrease the average laser power used by decreasing the laser pulse repetition rate and increasing the laser peak power. The use of laser pulse picker to mitigate thermal damage will be discussed in Section 31.5.

The processes that result in photodamage to the specimen are complex (Gu & Sheppard, 1995; König et al., 1996, 1997). For multiphoton excitation microscopy, oxidative photodamage is reduced by delivering the laser pulse energy into a small focal volume. With a confocal laser scanning microscope, more significant photodamage occurs, resulting from single photon absorption throughout the total volume. Since typical two-photon excitation microscopy requires milliwatt average power laser pulse trains consisting of nanojoule pulses, severe thermal damage can occur as a result of one-photon excitation. This situation is particularly severe for highly pigmented cells such as melanophores (Masters et al., 1999). Furthermore, the high peak power required for two-photon excitation may produce cell and tissue damage through dielectric breakdown mechanisms.

During a two-photon excitation process, it is important to realize that three-photon excitation can also be generated. Although the emission filters may be selected for fluorescence detection from the two-photon process, there may still be photodamage from the unintentional three-photon excitation process. When intentionally using three-photon excitation for tissue imaging, it is important to realize that three-photon excitation microscopy often requires an order of magnitude higher incident laser power than two-photon excitation in order to achieve similar rates of excitation. It is important to study the associated three-photon cellular damage mechanisms and identify damage threshold because of the high laser power required. Nevertheless, three-photon excitation microscopy provides a useful technique to excite those fluorophores with single-photon absorption bands in the ultraviolet and short-ultraviolet wavelengths.

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Electronic Spectroscopy, Laser Applications☆

W. Demtröder , in Encyclopedia of Spectroscopy and Spectrometry (Third Edition), 2017

Sensitive Absorption Spectroscopy

In classical absorption spectroscopy, the intensity I _T of a light beam transmitted over a pathlength L through a sample of absorbing molecules with absorption coefficient α (cm^{−   1}) is compared with the incident intensity I ₀. According to Beer's law of linear absorption

[1] $I_r=I_{0}exp\left(-\mathit{\alpha L}\right)\approx I_0\left(1-\mathit{\alpha L}\right)\mathrm{for}\mathit{\alpha L}\ll 1$

the absorption can be written as

[2] $\mathit{\alpha L}\approx \frac{I_0-I_{\mathrm{T}}}{I_0}$

The smallest still detectable absorption is limited by intensity fluctuations of the incident intensity I ₀ and by detector noise. The absorption coefficient α  = N_i σ_ik is the product of number density N_i of molecules in the absorbing level |i〉 and absorption cross section σ_ik (ν) for the transition |k〉   ←   |i〉. The minimum detectable number N_i of absorbing molecules

[3] $N_i=\frac{\mathrm{\Delta }{I}_{min}}{I_0\mathit{L\sigma }}$

is then given by the minimum difference ΔI _min  = I ₀  − I _T that still can be safely measured. For a given value of ΔI _min, the detection sensitivity can be increased by making the product I ₀ Lσ as large as possible. The various sensitive techniques of laser absorption spectroscopy attempt to minimize ΔI _min as well as to maximize I ₀ Lσ.

High-Frequency Modulation Spectroscopy

For high-resolution laser absorption spectroscopy, a narrow-band tunable CW laser can be used. Possible candidates are dye lasers, T_i : sapphire or other vibronic solid-state lasers, and the variety of tunable semiconductor diode lasers, which cover a spectral range from the blue to the mid-infrared. Of particular importance are the optical parametric oscillators that have been brought to reliable operation in the pulsed as well as in the CW mode with single-mode performance. They can now span the spectral region from 300 to 5000   nm.

When the output of such a tunable monochromatic laser is sent through an electrooptic modulator, the phase and thus the frequency of the transmitted laser wave is modulated and the frequency spectrum consists of the carrier and two sideband frequencies, where the sidebands have opposite phases ( Figure 1 ). At a modulation frequency of Ω  =   1   GHz, the separation of the sidebands from the carrier is approximately equal to the Doppler width of the absorption lines. If the laser wavelength is continuously tuned over the absorption range and the modulated laser beam is detected after transmission through the absorption cell, the output of a lock-in, tuned to the frequency Ω, will not show any signal as long as neither the carrier nor the sidebands overlap with an absorption line, since the beat signals between carrier and the two sidebands exactly cancel. Note also that any fluctuations in the laser intensity are automatically subtracted. If, however, one of the three frequencies coincides with a molecular absorption line, the exact balance is perturbed and a detector signal is generated. When the laser frequency ω is tuned over an absorption line, the measured signal profile S(ω) is similar to the second derivative of the absorption profile α(ω).

Figure 1. Principle of modulation spectroscopy. (a) Apparatus; (b) sidebands of phase modulation; (c) line profile of the signal S(ω).

Since the lock-in detectors now available cannot measure frequencies above 100   MHz, the output of the detector is heterodyned with a stable local oscillator of frequency Ω ₀ and the beat signal with frequency (Ω  − Ω ₀) is sent to the lock-in.

The sensitivity of this modulation spectroscopy is two to three orders of magnitude higher than in conventional spectroscopy and reaches a minimum detectable absorption of αL  <   10^{−   7}. Using multiple-path absorption cells (Herriot cells) and balanced detectors for detection of reference and transmitted beams, absorption coefficients as low as α  ≤   10^{−   10}  cm^{−   1} can still be measured.

Excitation Spectroscopy

Instead of measuring the transmitted laser intensity, the photons absorbed by the sample molecules can be directly detected because they excite the absorbing molecules into a state |k〉 that emits fluorescence ( Figure 2 ). The rate of detected fluorescence photons is

Figure 2. Level scheme of excitation spectroscopy.

[4] ${\dot{n}}_{\mathrm{FI}}={\dot{n}}_{\mathrm{abs}}\cdot {\eta }_{\mathrm{m}}\cdot \epsilon \cdot {\eta }_{\mathrm{k}}$

where ${\dot{n}}_{\mathrm{abs}}$ is the rate of absorbed laser photons, η _m  ≤   1 is the quantum efficiency of the excited atom or molecule, ε is the collection probability of the emitted fluorescence and η _k is the quantum efficiency of the detector. With 1   W incident laser power at λ  =   500   nm, a collection efficiency of ε  =   0.1, quantum efficiencies η _m  =   1 and η _k  =   0.2 and an absorption probability of αL  =   10^{−   15}, we obtain a photon counting rate of 60   photons   s^{−   1}, which gives a signal-to-noise ratio of 6 at a time constant of 1   s and a dark current of 10   photons   s^{−   1}.

For selected atomic transitions, the upper level may decay only into the initial lower level (true two-level system). In this case the same atom can be excited many times during its flight time T through the laser beam. If its upper-state lifetime is τ, the number of fluorescence photons emitted per second may be as high as T/τ (photon burst), if the laser is intense enough to saturate the transition.

Ionization Spectroscopy

The absorption of a photon in the transition |i〉   →   |k〉 can be also detected by ionizing the molecule in the excited state |k〉 by a second photon ( Figure 3 ). If the intensity of the second laser is sufficiently high, the ionizing transition may be saturated, which implies that every excited molecule is ionized. Since the ions are extracted from the ionization volume by an electric field that focuses them onto the cathode of an ion multiplier, the detection efficiency can reach 100%.

Figure 3. Level scheme of ionization spectroscopy.

This makes the technique very sensitive and in favorable cases single absorbed photons can be monitored. Since the first transition |i〉   →   |k〉 can be readily saturated, this also implies that single molecules can be detected.

In case of pulsed lasers, the laser power is sufficiently high to allow saturation of the two transitions even without focusing or with only weak focusing. If the ionization energy is high, two or more photons might be required for the ionizing step (resonant multiphoton ionization). With CW lasers, tight focusing is required to reach the necessary high intensity.

The further advantage of ionization spectroscopy is the mass-selective detection of the ions by means of a mass spectrometer. This allows the detection of specific isotopes in the presence of other species. With pulsed laser ionization, a time-of-flight mass spectrometer offers the best choice; for CW lasers, a quadrupole mass spectrometer is suitable.

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Laser Applications in Electronic Spectroscopy*

Wolfgang Demtröder , in Encyclopedia of Spectroscopy and Spectrometry (Second Edition), 1999

High-Frequency Modulation Spectroscopy

For high-resolution laser absorption spectroscopy, a narrow-band tuneable CW laser can be used. Possible candidates are dye lasers, Ti:sapphire or other vibronic solid-state lasers, and the variety of tuneable semiconductor diode lasers, which cover a spectral range from the blue to the mid-infrared. Of particular importance are the optical parametric oscillators that have been brought to reliable operation in the pulsed as well as in the CW mode with single-mode performance. They can now span the spectral region from 500 to 5000   nm.

When the output of such a tuneable monochromatic laser is sent through an electrooptic modulator, the phase and thus the frequency of the transmitted laser wave is modulated and the frequency spectrum consists of the carrier and two sideband frequencies, where the sidebands have opposite phases (Figure 1). At a modulation frequency of Ω=1   GHz, the separation of the sidebands from the carrier is about equal to the Doppler width of the absorption lines. If this modulated laser beam is detected after transmission through the absorption cell, the output of a lock-in, tuned to the frequency Ω, will not show any signal as long as neither the carrier nor the sidebands overlap with an absorption line, since the beat signals between carrier and the two sidebands exactly cancel. Note also that any fluctuations of the laser intensity are automatically subtracted. If, however, one of the three frequencies coincides with a molecular absorption line, the exact balance is perturbed and a detector signal is generated. When the laser frequency ω is tuned over an absorption line, the measured signal profile S(ω) is similar to the second derivative of the absorption profile α(ω).

Figure 1. Principle of modulation spectroscopy. (a) apparatus; (b) sidebands of phase modulation; (c) line profile of the signal S(ω).

Since lock-in detectors now available cannot measure frequencies above 100   MHz, the output of the detector is heterodyned with a stable local oscillator of frequency Ω₀ and the beat signal with frequency (Ω−Ω₀) is sent to the lock-in.

The sensitivity of this modulation spectroscopy is two to three orders of magnitude higher than in conventional spectroscopy and reaches a minimum detectable absorption of αL  <   10⁻⁷. Using multiple-path absorption cells (Herriot cells) and balanced detectors for detection of reference and transmitted beams, absorption coefficients as low as α≤10⁻¹⁰  cm⁻¹ can still be measured.

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Single-Molecule Enzymology: Nanomechanical Manipulation and Hybrid Methods

V. Belyy , A. Yildiz , in Methods in Enzymology, 2017

2.1 Optical Layout of the Trap

Our trapping microscope consists of a 2-W 1064-nm fiber-coupled CW laser (Coherent Compass) for trapping beads, a pair of acousto-optical deflectors (AODs; AA Opto Electronic DTSXY-400) for precise beam steering, and a 1.49 NA oil immersion objective (Nikon CFI Apo TIRF 100   ×) which focuses the trapping beam to a nearly diffraction-limited spot in the image plane. The trap utilizes back-focal plane interferometry for position detection (Gittes & Schmidt, 1998a; Tolić-Nørrelykke et al., 2006) by imaging the trapping laser's beam onto a 200-kHz position sensitive detector (PSD; First Sensor DL100-7-PCBA3). Fluorescence laser lines are available for TIRF imaging, and a separate 845-nm solid-state laser is reflected off the coverslip and projected onto a separate PSD for long-term focus stabilization. Precise power control is achieved by discarding the required percentage of the laser's output into a beam dump, as determined by a half-wave plate mounted on a motorized rotary mount followed by a polarized beam-splitter cube. Several excellent reviews and guides have been written on the general layout and calibration of optical traps (Bustamante, Chemla, & Moffitt, 2009; Neuman & Block, 2004). We will instead focus on often-overlooked aspects of optical trap design—its custom-written control software and sources of experimental noise in trapping measurements.

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